I've started to run into the same problem that many other 'quant' type people have run into. I seem to get stuck on 'cracking' specific problems during my gmat tests (the variety of question seems to vary, but the one consistent problem is that they seem to be the difficult type questions). In any case, I end up running a string of 4-6 minute problems, and that ends up destroying my pace. The issue is that in un-timed environments I can typically push through difficult problems when I'm thinking clearly, but I get into a bit of a panic once I see a problem that I can't crack.
Does anyone out there have a collection of timed problem sets?