When doing 1 to 1 nating I am getting a lot of dropped packets

Ok I am having a problem with my internet and I have gone on with our internet provider for over a year and they are down to it is something with our equipment not on their side.  Here is what is going on we use Cox Communications for our cable internet we have 150Mbps download and 20Mbps upload.  We are trying to use an untangle box for the router and I am trying to do 1to1 nating on it from 3 internal ips to 3 external ips.  I also have to ports forwarded from the external ips to the internal ips.  we had this same setup with an sa520 before and it worked just fine then about a year ago we had problems and we though it was the router because we have had problems with the router at other sites so I put the untangle in place.  if I have a switch on the cable modem hooked up to 3 seperate routers with each one having the external ip and set the server to use it as a gateway then it works just fine but if I try to use just the untangle box to do all the routing then we get random packet drops.  what I have done to test this is ping the gateway on cox's side and also ping the modem ip and when the pings drop to the gateway at cox they do not drop to the modem and to make it drop even more if I do a speed test as soon as it hit the upload side of the test we get 5 to 6 pings dropped and the test will not go through or show an upload speed of like 1Mbps if we are lucky.  Cox tried a netscreen router which they say they do what I am trying to do with those all the time and when we hooked our network up to it it had the same results.  They say when we hook the untangle up to do all the routing it is not doing broadcast packet filtering or they say it could be a DOS fuction problem.  But either way they say it is not anything on their end it is something with our equipment.

Sorry for the long rant but I wanted to try to say everything we tried if anybody has any ideas please let me know