22r doesn't have a timing belt. I'm sure you mean chain. If it's actually slipping, you have tensioner issues and possibly a very old stretched chain. I'd replace them.

 

Chain, correct. The chain was replaced at the rebuild. The second time this happened, it trashed the timing cover. Cover, chain, tensioner were replaced.

 

I'd wonder if it was installed properly, did the same person/shop do it each time?

The only other thing I can think of is the oil passage to the hydraulic tensioner is clogged, so the tensioner isn't working properly or at all.

 

I've had a bunch of 20R and 22R engines apart. Not much besides those parts can cause this. The guides all need to be replaced in those engines when the chain is done. The tensioner as well. If they are all functional, there really isn't much else to go wrong. You do need good oil pressure but the chain jumping is really odd.

Possible that the chain is stretched or just a bad chain I guess.

Edited your title so maybe more folks will see and chime in.

 

The tensioner has a lock position to make the chain easier to install. Was it ever released from that position? The tensioner is spring loaded and should immediately put basic pressure on the chain. If it was not 'unlocked' I can definitely see you having the problem you are experiencing.

 

Who sais it's jumping?, and who is fixing it when it jumps?

If this is whats happeneing, then it sounds like someone saved money on parts.
Sprockets, chain, tentioner all come in the kit.