[Surestick]

Definitely clean & lube it.
Should be easy to check if its leaking - attach a full tank, let it sit for half an hour without touching it & see if the pressure changes.

If neither of these help I would suggest timing it. As mentioned above, the stock bolt doesn't seal the air coming in to the dump-chamber when the bolt is forward. If your dwell is too long you will be dumping extra air down the barrel with every shot. Apparently after market bolts that do seal the dump-chamber don't have much of an effect on efficiency on a properly timed Ion so I wouldn't worry about that unless you want to get the absolute best efficiency out of it.

Nicad (of Deadly Wind fame) posted the following procedure to set dwell on the Ion over on the Tinkers Guild tech forum: " The "optimized" dwell was set by finding the lowest dwell that didn't affect the velocity, then adding one for consistency" Tinkering Tech Forums :: View topic - Shot count data on a bone stock Ion. Twisted results??

If all this fails start shooting less! ;-)