Thats whats confusing me about the whole thing, I'm loading all the data into memory, and processing it from there - no database connections, no reading from the hard-drive, the section being timed only uses data in memory.

Other programs have no trouble getting all 4 cores at 100%,so I presumed it was something in .net that was limiting it.

I'm wrote a C# application to run a batch process on around 50 million records. Initially this was taking around about 60 seconds to run. Since I'm running a multi-core machine (Intel Quad 6600 - 4 cores), I alterered the code to use 4 threads, which had the desired effect of speeding up my code to around about 40 seconds running time, with all 4 cores running at 100%..

Now, my problem is this. During a routine upgrade, I replaced...