ORIGINAL: bla bla

If it's a constant surging, up and down with the RPM's for no aparent reason, is a classic YS problem.
Normally that means escaping air between the ring and the liner, very popssibly due to the fact that things aren't steating properly due your strip down. Do a ring change and it should go away for a while confirming the problem. It could very likely come back within 50 runs confirming you'll need to change the ring and the liner, better still replace the ring, piston and liner. It's a classic problem. Better still just send it to YS performance and have a complete strip down rebuild and have them run it up. they can do a small gas porting mod to the piston to help get a better ring/liner seating. You'll have what esentially a brand new engine.
Try what was suggested in the post above, hopefully it'll work, if it doesnt. get a ring.