>yes when pedaling and the chain is on tenth gear the pedals jump
forward a little ,the more presure the more it jumps<

So it is not changing gears but just skipping teeth.

Main causes for this are likely a worn sprocket and chain or new chain
on worn sprocket.

Also look at the amount of chain wrap. Insufficient could cause a jump.
You may need to adjust the "fore and aft" screw on deraileur (has a name
but I can't recall. ) to try to get a bit more chain wrap. Screw it out
but not so far that the little idler cog touches the sprocket.

The final thought is frame flex although this would more likely tend to
cause a change in gears.

John Lewis