Its a fairly common problem, however the fact that you NEEDED to install a diode indicates you definetly have problems. Just because there is no check engine light does not mean you have codes. In fact if you hit limp mode you probably have codes stored for "a difference in pressure deviation" meaning most likely you have a boost leak somewhere or less likely a faulty MAP (maybe even MAF sensor). I'd remove the diode and perform a boost leak test (common place is the intercooler connection near the pass fender.
Surging can under full throttle can still happen, I swtiched to a N75j valve and opened the wastegate on the turbo to help aleviate the problem. But I would start with my first suggestions, no reason a diode is required on a chipped ECM.