[dpvu]

Start with the hop up. Disassemble the unit and clean the bucking as others have suggested. There could be excess oils getting in there if the mechbox was overlubed and the lube used for the cylinder/piston is too light.

It doesn't sound like an air seal issue so I wouldn't open the mechbox right now if you're not comfortable with that. Excess oils will eventually all be expelled and you may just need to clean your hop/barrel more often.