It is on nitrogen and it will shoot great for a couple games then lob hard or have a significant reduction on fps. Clean the reg and mess around with the velosity adjustment an brings it back for awhile. Really odd.
try a different reg and see if the same thing happens, if it does is your velocity going down with the psi of the tank? And have you tried maybe looking at the valve? Have u tried maybe using a lighter valve spring or a harder main spring.