Run the router fully up so the shaft is locked before you do much of anything. Foil is pretty soft, I'd concentrate on the fixes offered before relying on a shim. If the bit has always worked fine, then something else has happened, perhaps the height lock isn't fully tightened? The Triton is a pretty powerful machine and the resistence of the wood and a slightly loose height lock could result in creeping. I'd be very hesitant to spread the muscle chuck before doing all the other suggested checks.
The more I do, the less I accomplish.