I'd start by getting the codes read to see if anything shows up, then check the valve clearances/ CO readings, [measured at each exhaust port] and the throttle switch often overlooked on older cars it is an Inspection two service item on S50 M3's
The TPS is new, valve clearences just done, but I will put my diagnostic software on it and see what is comes up with.
I'm more interested that I've read a few people have had the same problem on here with their s50 converted cars and would like to know if they have solved it and what it was?
Mine is hesitating at WOT, not part throttle. So we don't have the same problem.
Try to unplug you MAF while the engine is running, you shall hear a small change in the idle. If you don't the MAF is toasted.