A transient failure has occurred. exchange 2007 event id 4001

hi expert
i installed exchange 2007 to work with existing exchange 2003, the 2007 was installed as a member server same as the 2003 is,no hub transport has been configure on the new exchange , all incoming and outgoing emails are routing thru exchange 2003, the 2007 data store is 80 gig and the 2003 data store is 14 gig
my problem is that every day at the end of business day the store.exe on the 2007 get overloaded, by looking on windows perfomance i can see 5 gig of memory use and this server has only 4 phisical memory, the info store process is working on 5 gig memory use, from that point emails stop flow into the users mailboxes,when i restart the info store everything go back to normal, the phisical memory use ia back to 2 gig for couple of hours and everything is working,the only event i see every time on the application events on the exchnage 2007 is that one

A transient failure has occurred. The problem may resolve itself in a while. The service will retry in 56 seconds. Diagnostic information:

Cannot open mailbox /o=Almar Sales/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=ALMARNYEXCH2/cn=Microsoft System Attendant.
Microsoft.Exchange.Data.Storage.ConnectionFailedTransientException: Cannot open mailbox /o=Almar Sales/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=ALMARNYEXCH2/cn=Microsoft System Attendant. ---> Microsoft.Mapi.MapiExceptionLogonFailed: MapiExceptionLogonFailed: Unable to open message store. (hr=0x80040111, ec=-2147221231)
Diagnostic context:
    Lid: 18969   EcDoRpcExt2 called [length=252]
    Lid: 27161   EcDoRpcExt2 returned [ec=0x0][length=116][latency=0]
    Lid: 23226   --- ROP Parse Start ---
    Lid: 27962   ROP: ropLogon [254]
    Lid: 17082   ROP Error: 0x80040111
    Lid: 26937  
    Lid: 21921   StoreEc: 0x80040111
    Lid: 27962   ROP: ropExtendedError [250]
    Lid: 1494    ---- Remote Context Beg ----
    Lid: 26426   ROP: ropLogon [254]
    Lid: 22970  
    Lid: 8620    StoreEc: 0x80040111
    Lid: 10786   dwParam: 0x0        Msg: ALMARNYEXCH2
    Lid: 1750    ---- Remote Context End ----
    Lid: 26849  
    Lid: 21817   ROP Failure: 0x80040111
    Lid: 26297  
    Lid: 16585   StoreEc: 0x80040111
    Lid: 32441  
    Lid: 1706    StoreEc: 0x80040111
    Lid: 24761  
    Lid: 20665   StoreEc: 0x80040111
    Lid: 25785  
    Lid: 29881   StoreEc: 0x80040111
   at Microsoft.Mapi.MapiExceptionHelper.ThrowIfError(String message, Int32 hresult, Int32 ec, DiagnosticContext diagCtx)
   at Microsoft.Mapi.ExRpcConnection.OpenMsgStore(OpenStoreFlag storeFlags, String mailboxDn, Guid mailboxGuid, Guid mdbGuid, MapiStore msgStorePrivate, String& correctServerDn, ClientIdentityInfo clientIdentityAs, String userDnAs, String applicationId, CultureInfo cultureInfo)
   at Microsoft.Mapi.ConnectionCache.OpenMapiStore(String mailboxDn, Guid mailboxGuid, Guid mdbGuid, ClientIdentityInfo clientIdentity, String userDnAs, OpenStoreFlag openStoreFlags, CultureInfo cultureInfo, String applicationId)
   at Microsoft.Mapi.ConnectionCache.OpenMailbox(String mailboxDn, Guid mailboxGuid, Guid mdbGuid, WindowsIdentity windowsIdentityAs, String userDnAs, OpenStoreFlag openStoreFlags, CultureInfo cultureInfo, String applicationId)
   at Microsoft.Exchange.Data.Storage.ConnectionCachePool.OpenMailbox(String serverDn, String userDn, String mailboxDn, Guid mailboxGuid, Guid mdbGuid, Object identity, ConnectFlag connectFlag, OpenStoreFlag openStoreFlag, CultureInfo cultureInfo, String clientInfoString, Boolean secondTry)
   --- End of inner exception stack trace ---
   at Microsoft.Exchange.Data.Storage.ConnectionCachePool.OpenMailbox(String serverDn, String userDn, String mailboxDn, Guid mailboxGuid, Guid mdbGuid, Object identity, ConnectFlag connectFlag, OpenStoreFlag openStoreFlag, CultureInfo cultureInfo, String clientInfoString, Boolean secondTry)
   at Microsoft.Exchange.Data.Storage.ConnectionCachePool.OpenMailbox(String serverDn, String userDn, String mailboxDn, Guid mailboxGuid, Guid mdbGuid, Object identity, ConnectFlag connectFlag, OpenStoreFlag openStoreFlag, CultureInfo cultureInfo, String clientInfoString, Boolean secondTry)
   at Microsoft.Exchange.Data.Storage.ConnectionCachePool.OpenMailbox(String serverDn, String userDn, String mailboxDn, Guid mailboxGuid, Guid mdbGuid, Object identity, ConnectFlag connectFlag, OpenStoreFlag openStoreFlag, CultureInfo cultureInfo, String clientInfoString)
   at Microsoft.Exchange.Data.Storage.MailboxSession.Initialize(LogonType logonType, ExchangePrincipal owner, ADOrgPerson delegateUser, Object identity, OpenMailboxSessionFlags flags)
   at Microsoft.Exchange.Data.Storage.MailboxSession.CreateMailboxSession(LogonType logonType, ExchangePrincipal owner, ADOrgPerson delegateUser, Object identity, OpenMailboxSessionFlags flags, CultureInfo cultureInfo, String clientInfoString)
   at Microsoft.Exchange.Data.Storage.MailboxSession.OpenAsAdmin(ExchangePrincipal mailboxOwner, CultureInfo cultureInfo, String clientInfoString)
   at Microsoft.Exchange.Servicelets.SystemAttendantMailbox.Servicelet.Work()

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our network is mixing with 100MB swiches and 1gig swiches, on the 2007 i have two NIC`s card, the one is connected is a 100MB link
do you guys think im having an hardware overloaded isue? i mean maybe i transered too many mailboxes from the 2003 to the new machine(2007)? or maybe this is a network isue?
thanks