Hi Shiz, I've just seen your post tonight. First of all can I say thanks so much for taking the time to write such a detailed and informative post. I actually sent her back to LWA to fix the problem and to do the spring upgrade at the same time. They have had it nearly 2 weeks. I did email them last week to ask about progress and they said they had been busy but the work would be done sometime last week. I still haven't heard anything......
Anyway, back to the other issues, when I said sticking on full auto, I mean it didn't stop firing when I released the trigger , not the gearbox locking. I didn't notice any water in the stock but it's possible there was.......I've read a few things about the gen 3 mostest being shat and burning out even totally dry! The battery's where totally fine. I always swap at lunch and so it had barely been used.....I even tried it with a fresh battery when I got home just to check but with. The same problem.
I think I do need to change the mosfet by the sounds of it, and everything else you said sounds like the sensible way to check but it does sound terribly daunting to me as I've only been playing a few months and have virtually zero knowledge and experience of dismantling a gun. Don't want to learn the hard way on a £320 rifle. I guess I'm going to have to learn if it's going to take weeks for lwa to sort it out!
Cheers for your help