I'd start with getting a look underneath. Compare it with the left flipper, are all the wires attached, are all parts securely tightened to where they should be, when you move the flipper with your hand are the switches all making contact like they should, etc. Specifically, I'd check that the screws on the leaf switches are all tight based on a SG I once had exhibiting similar behavior.

I'm not really sure what it could be, but I find the above leads me to the solution in most cases.