Do you know how old the gun is?
first thing i would do is Make sure its all air tight. cilinder, cilinder head, nozzle and bucking to barrel seal. Seems like that could be the issue.
If its not that i would start looking at the air nozzle, cilinder head or tappet plate for wear. if it is an older gun that shit could wear out.
Then after checking all of that and it was all good, still shooting with same results, i'd replace the bucking make sure its not warn causing the problems.
Another one would be to check the hop up nub. Alot of people think you can just drop in a H nub and get a huge gain in accuracy from that.. sadly thats not how it works.. thats just going to cause it to eather jam, or tear the bucking.
causing simular problems to what you have.
try just lubing the hell out of the barrel with some silicone spray first and see if it still has same results