results differ when testing full range of tests and subset o

I'm using NCover 1.5.4 beta, NUnit 2.2.8, NCoverExplorer 1.3.4.1702 & Testdriven.Net (latest).

I have a suite of NUnit tests, about 14 classes in all, that test various bits of my applications functionality.

Assume they are named TestClass1 thru 14, testing Appclasses 1 thru 14.

If I run the whole suite, I am given a %complete of 54%, with AppClass1 being 78% covered.
When I just run TestClass1 (which tests AppClass1), I get a coverage figure for AppClass1 of 93%, with code being shown as exercised that dosn't get show when running the whole suite.

I can show the code is being run quite happily by attaching debuggers to the appropriate processes, and can trace through the code (which it then reports as not being exercised...)

Am I experiencing normal behaviour or is this a bug? (known or otherwise..)

I can't put the code under test, or the NUnit code on here for various reasons, but any help would be appreciated...

Thanks in advance

Hugh