It is rather unusual that your rifle has gained power, usually it is the other way around, especially if you haven’t been inside the gearbox or changed the hop rubber (which if you have, might have improved the air seal and given you the power boost). Also a variance of 30FPS is very high.
My first suspicion is that you have got a bottle of mixed weight BBs - the lighter BBs will be giving you you the higher FPS numbers, and the heavier ones the lower output. But if you are certain that this is not the case, and you haven’t mixed up your BBs with some from a variety of sources, then probably the next thing I would do would be to swap out the hop rubber/bucking. If you haven’t done this before, this is a very easy task - plenty of YouTube videos on it (the process for your gun will be the same as any other 'M4' type, so search for that rather than your exact model of gun, which would probably not give you as many/any hits). Maple Leaf MR Hop AEG rubbers (yellow/60d degree) is my preferred choice, combined with an omega nub. But these don't fit particularly well in all hop chambers/guns. A slightly less well performing but still good choice (and one that seems to fit in a wide variety of hop units) will be the G&G green hop rubber.
