The chain on my bike has just recently stating to suck, the chain seems to suck from underneath on the front rings mainli in lowest and middle ring, I have noted the rear mech is jumping forwards occasinaly. The rear mech is old but in good condition, it only happens when i am riding, not if off bike and spinning cranks.
I cannot work out what is casing it any idea's ST massive.

Thanks

does this happen every revolution of pedaling??? if so check the chain for a stiff link if found to be this free up the problem link by gently prising apart with a suitable screwdriver this will free up link and stop the jumping
hope this helps good luck

How clean is your chain and chain rings? I suspect that the chain and rings are worn excessively. May well be time for new chain/cassette and chain rings.

Your drivetrain may well be worn out, the rings are hooking your chain as it is too long and the teeth are worn into shark fins and the same goes for the rear, the spaces between links have worn too long and they occasionally gather up and skip forward. Time to lay out some cash.

The drive train is not worn to badly, I may check the links on the chain

Normally chainsuck is a worn middle-ring. May not look worn, but it doesn't take much TBH. Depending on how old everything is (or to be sure use a chain checker), consider also changing the chain and the cassette at the same time.

Often caused by burrs on the chainring teeth too, you'll sometimes eke a bit more life out of them by tickling them with a small round file to remove the offending bits.