It could be the MOSFET - possibly overheating? - may explain why it starts off ok.
The blokes at Pro Airsoft were saying that they'd had a number back with blown mosfets and that another large retailer (cant remember which one they said, possibly Zero One) had got a ton of mosfets shipped from G&G to replace faulty ones.
Pro Airsoft are in the process of developing a drop in replacement fet unit.
In any case, I have a Gen 3 running on 9.9v life batteries, seems to be running OK so far after 2 skirmishes (I fire semi auto most of the time). Time will tell I suppose.