Do the rams leak down when you leave them up? If they do leak down (if you plug the ram ports) then you have internal leakage in the rams seals.
I guess you could eliminate the valve by connecting a pressure guage to the work port of the valve and check that is is getting full pressure. If the other rams are working fine them you really only have the options of a worn valve spool or a leaking ram.
Its quite do-able to rebuild rams yourself, I've not done one myself, but apart from needing a bit of brute force and some large tools I understand its quite straight forward. On most, or the ones I have come across atleast, you have a nut on the rod end of the ram, you undo that nut and the rod and seals should slide out. Then you have a nut on the end of the rod that you undo to take the seals off. You can buy seal kits based on the rod and bore size of the ram, I think the material of the seal is also a factor, but the supplier for the kit would probably be able to offer guidance with that.
I'm sure that there is someone here who has rebuilt rams who knows more.