Shimano Revoshift repair
It depends on how much you value your time. Taking one of these to pieces and putting it back together again is do-able but may not give you the fix needed, whereas, buy the new part and it might save you time.

To give it a go, type in 'revoshift' into http://techdocs.shimano.com/techdocs/index.jsp and pull up your manual.

And personally, if I were you, I would give it a go.