MikMo:
I once had a problem like that with my 2n3055-based h-bridge that I have breadboarded; I kept having a problem where I could get the motor to turn one way, but not the other. I figured the maybe one of my transistors were fubar'd, so I swapped a couple around - and I still had the problem!
WTH? I started thinking my breadboard was working right (or I had a short somewhere); I tried everything for a few hours - what was even more maddening was the sometimes (but only briefly!) the circuit would work - then stop; sometimes swapping the transistors would help - it didn't make any sense.
It wasn't until I decided to try out a different set of aligator test leads (which I used to connect the collectors together on the TO-3 cases) did I find out, and confirm with a multimeter, that one of them had an intermitent break near the clip - once that was sorted, I resoldered the clip, and haven't had a problem since.
;D