Just to back up what some have already said
1) Don't adjust the reg for fine tuning velocity.
2) The IVG uses a 1/8" allen or you could remove the cocking rod an use a slightly larger wrench (not sure what size)
3) Make sure your paint matches the barrel. If it doesn't and a ball rolls part way down the barrel you would certainly see that kind of behavior.
4) Make sure you have fresh batteries in the gun...oh wait...nevermind.
The S6's are pretty robust so I would suspect a barrel match issue but cleaning and lubing certainly wouldn't hurt.