My problems with your problem:
1: a M130 spring shouldn't give you 425 w/ .25's. It should give you around 425 w/ .20's. An SP130 should put you near, but still under that level of performance unless you have perfect airseal, in which case it will approach it.
2: It's hard to find anywhere that will let you run that hot (425 w/ .25's) full auto.
That being said, there aren't a lot of things that could be causing this. I would say it's your motor, or your battery, period.
If it cycles at all, then your piston isn't too big. If it lasts a few shots, or 20 or so in this case, then something is lagging and either heating up or reducing output in the electrical system.
Chances are that your motor is overheating from the load and the lipo. Each time it fires, it heats up a little more. Every time it heats up some, it gets a little less efficient and has to work harder to fire the next shot... which heats it up even more... this cycle continues until it can't work as hard as you need it to and voila, the gearbox slows down then locks up.
Same idea if the lipo isn't cutting it: It loses some voltage, heats up a little bit, then can't provide as much power and needs to provide more because it's internal resistance goes up as it heats up, and on and on in it's own heat cycle. This is less likely to be the problem than the motor though.
It doesn't sound like you know too much about the gun and how to upgrade it... so I'm assuming that you're just saying 'the motor is good' because it can fire those first few shots... well, it's very likely the actual problem.
