I think I solved the problem - for anyone who might be having a similar issue, I'll post the solution:

My processAffinityMask was set to 15 (distribute load over 4 processors) - I simply changed it to 255 (i7 with HT) which solved the problem. My old quad core (q6600) liked the 15 setting, but i guess my i7 likes 255 because of the hyperthreading. Anyway, problem solved and I'm happy - getting good great frame rates :-)