Before you do something that might cause major damage to the drive, I'd like to suggest that you telephone Broadway Limited customer service and ask for their help.
They'll probably ask you to return it to them for service.
Not sure how to check this engine to see if the gears are slipping. What is a press fit universal?
You mentioned that you "oiled it"... herein may contribute to your problem.
Many of the mechanical connections-- motor shaft to universal, axle gears to the axle, are just press fit. Even with "plastic compatible" oils they may make the press fit joints just loose enough to cause slippage between plastic gears and metal shafts. And once it slips a little, it only gets worse.
Be ever so SPARING on lubrication. Less is more!
Check any of the components where you have a press fit universal, it may have split allowing the motor shaft to spin. You'll have to replace it. Even ACC won't repair the bond... for long.
Ed