My mower blades won't engage fully on my Gravley 14-G?

I have a Mid.1990's Gavely 14-G Professional.When I engage the mower blades they will only half engage or barely engage.Then in a few minutes they may fully engage.Then when you drop the deck down on the grass the blades will slowdown or stop.I tried adjusting the pto lever with no luck.I don't know if its a clutch or electrical.The way it kick in and out it acts like it might be electrical.Please help.Thanks