[Haskell-cafe] debugging a hanging program: where to start?

Hi Cafe, I have a program that I can reliably cause to hang. It's concurrent using STM, so I think it could be a deadlock or related issue. I also do some IO, so I think it could be blocking in a system call. It only hangs when compiled with -threaded. I tried building with -prof, and running with -hc -xt to get a clue where in which function it is stopping, but the resulting profile didn't help much. I've started sprinkling print statements, but I thought I'd trawl for better tips and techniques. Any suggestions on approaches? Thanks, Aran