Cat 325L drive slow on one side

My Cat 325L (9kl00122) is slow on one side. I replaced the swivel seals a while back and this made no difference.
I noticed yesterday when I lift the slow side in the air and run the track forward or reverse and push down on the stick, the track speed goes to normal. The stick doesn't lift the digger until I stop moving the track. What could be the cause of this?
Any help would be great.

Cheers
Leigh

Cat 325L drive slow on one side