Thanks Martyn, I had allocated a button on FSUIPC to APU STARTER. Moving the power to idle was causing the power to trip inflight when the switch was in the APU ON position. Switching this off has cured the problem! I tried this several times, APU OFF keeps the electrics online, APU ON and it trips out everytime. Also probably explains why this only happened in the air - there was no problem while taxying. Thanks for the support!