I know its not nice to say but glad to see its not just me and the old APUs having problems.
We got the GTCP36-4A and we alleviated a lot of problems by changing oil filter element regularly (expensive but worth it) and keeping a close eye on the surge valve.
But I have yet to get the APU to meet its overtime limit of 3000 hours operation. We run them on condition and when they go U/S we change the APU. We have had APUs about 200 hours out of third party workshop, deciding to throw their impellers all over the place.
For us the APU is the weak link in our operation.
I believe reearch is well advanced on fuel cell powered APUs, but we will wait and see.