Keyset does not exist WebApi X509 Self Signed Certificate

HI All,

I have created a self signed certificate for signing my JWT tokens for access to a web api.

Now (i think as my machine has restarted but i'm not 100%) I am getting the error;

System.InvalidOperationException was unhandled by user code
  HResult=-2146233079
  Message=IDX10614: AsymmetricSecurityKey.GetSignatureFormater( 'http://www.w3.org/2001/04/xmldsig-more#rsa-sha256' ) threw an exception.
Key: 'System.IdentityModel.Tokens.X509AsymmetricSecurityKey'
SignatureAlgorithm: 'http://www.w3.org/2001/04/xmldsig-more#rsa-sha256', check to make sure the SignatureAlgorithm is supported.
Exception:'System.Security.Cryptography.CryptographicException: Keyset does not exist

   at System.Security.Cryptography.Utils.CreateProvHandle(CspParameters parameters, Boolean randomKeyContainer)
   at System.Security.Cryptography.Utils.GetKeyPairHelper(CspAlgorithmType keyType, CspParameters parameters, Boolean randomKeyContainer, Int32 dwKeySize, SafeProvHandle& safeProvHandle, SafeKeyHandle& safeKeyHandle)
   at System.Security.Cryptography.RSACryptoServiceProvider.GetKeyPair()
   at System.Security.Cryptography.RSACryptoServiceProvider..ctor(Int32 dwKeySize, CspParameters parameters, Boolean useDefaultKeySize)
   at System.Security.Cryptography.X509Certificates.X509Certificate2.get_PrivateKey()
   at System.IdentityModel.Tokens.X509AsymmetricSecurityKey.get_PrivateKey()
   at System.IdentityModel.Tokens.X509AsymmetricSecurityKey.GetSignatureFormatter(String algorithm)
   at System.IdentityModel.Tokens.AsymmetricSignatureProvider..ctor(AsymmetricSecurityKey key, String algorithm, Boolean willCreateSignatures)'.
If you only need to verify signatures the parameter 'willBeUseForSigning' should be false if the private key is not be available.
  Source=System.IdentityModel.Tokens.Jwt
  StackTrace:
       at System.IdentityModel.Tokens.AsymmetricSignatureProvider..ctor(AsymmetricSecurityKey key, String algorithm, Boolean willCreateSignatures)
       at System.IdentityModel.Tokens.SignatureProviderFactory.CreateProvider(SecurityKey key, String algorithm, Boolean willCreateSignatures)
       at System.IdentityModel.Tokens.SignatureProviderFactory.CreateForSigning(SecurityKey key, String algorithm)
       at System.IdentityModel.Tokens.JwtSecurityTokenHandler.CreateSignature(String inputString, SecurityKey key, String algorithm, SignatureProvider signatureProvider)
       at System.IdentityModel.Tokens.JwtSecurityTokenHandler.WriteToken(SecurityToken token)
       at TelematicusApi.ProvidersMyJwtFormat.Protect(AuthenticationTicket data) in c:\Users\bernie\Documents\Visual Studio 2013\Projects\Api\Providers\MyJwtFormat.cs:line 57
       at Microsoft.Owin.Security.Infrastructure.AuthenticationTokenCreateContext.SerializeTicket()
       at Microsoft.Owin.Security.OAuth.OAuthAuthorizationServerHandler.<InvokeTokenEndpointAsync>d__22.MoveNext()
  InnerException: System.Security.Cryptography.CryptographicException
       HResult=-2146893802
       Message=Keyset does not exist

       Source=mscorlib
       StackTrace:
            at System.Security.Cryptography.Utils.CreateProvHandle(CspParameters parameters, Boolean randomKeyContainer)
            at System.Security.Cryptography.Utils.GetKeyPairHelper(CspAlgorithmType keyType, CspParameters parameters, Boolean randomKeyContainer, Int32 dwKeySize, SafeProvHandle& safeProvHandle, SafeKeyHandle& safeKeyHandle)
            at System.Security.Cryptography.RSACryptoServiceProvider.GetKeyPair()
            at System.Security.Cryptography.RSACryptoServiceProvider..ctor(Int32 dwKeySize, CspParameters parameters, Boolean useDefaultKeySize)
            at System.Security.Cryptography.X509Certificates.X509Certificate2.get_PrivateKey()
            at System.IdentityModel.Tokens.X509AsymmetricSecurityKey.get_PrivateKey()
            at System.IdentityModel.Tokens.X509AsymmetricSecurityKey.GetSignatureFormatter(String algorithm)
            at System.IdentityModel.Tokens.AsymmetricSignatureProvider..ctor(AsymmetricSecurityKey key, String algorithm, Boolean willCreateSignatures)
       InnerException: 

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which is being thrown here;

            var token = new JwtSecurityToken(
                _issuer,
                audienceId,
                data.Identity.Claims,
                issued.Value.UtcDateTime,
                expires.Value.UtcDateTime,
                x509SigningCredentials);

            var handler = new JwtSecurityTokenHandler();

            var jwt = handler.WriteToken(token);

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This has happened before and following this link fixed the issue. However, it just seem to be short term.

http://stackoverflow.com/questions/12106011/system-security-cryptography-cryptographicexception-keyset-does-not-exist

Can anyone help me to solve this permanantly?