Had an almost identical problem with my 32 gts-t, fine when it was cold but once it had warmed up or if it was a hot day the revs would just do whatever the hell they wanted, hunt like crazy, idle at a constant 2000rpm, or just drop straight to 0 and stall (this being the most common). However mine wasn't an electrical problem, took it into ASG who told me i needed to replace the stock ECU, my injectors were leaking and something about the AFM being wrong. Probably not a huge help to you but something to make sure isn't the case I guess.