Check the status of the small gear on the motor, particularily the setscrew there and the gear teeth. If it is slipping, you’ll seee that.
The nut that is embedded inside the idler arm will spin in the socket and strip all the material out if you even look at it funny. If you can avoid taking it out, I would do so. Chances are you will not be able to get it out without cracking the Idler arm, so make sure you have a new one on hand. That being said, if the idler arm filliament notch is in decent shape, which it looks like it is from your picture, there is no reason to remove it since you can get to the arm mechanicals without doing so.
if the small gear isn’t the issue, check the idler arm bearing itself to see if that still rotates freely, and check the hobbed bolt for debris and gummed bearings. Also check that the nut on the end of the hobbed bolt hasn’t loosened. it should be just tight enough to prevent any forward or backwards movement of the bolt, but loose enough that the bolt turns freely with no drag. If you have ever sprayed lubricant in there, you may want to dissassemble and clean the whole thing.