A few things here.
"Everything I know" I learned from
www.phonoland.com
Check the underside of the turntable where the jockey wheel turns on it to ensure it's free from any grease. They are pretty robustly constructed so unless someone has dropped one very hard on its edge it's unlikely to be out of true.
The jockey wheel as has been mentionioned is another possible suspect and possibly a weak or loose retaining spring. If the wheel is worn or has flat spots it might slip a bit but would only cause slight "wowing."
I think I paid £10 for a new one, but parts are more expensive over here in England.
The three rubber motor support grommets, if they're original they are usually black. If they are yellow they are more likely to be replacements. I doubt if that is the problem unless they've perished completely. What does happen after time, is that they become hard and transfer the vibration of the motor to the turntable assembly and particularly the tone arm. This causes background noise when playing a record. This can be determined by lightly placing a finger on the point of the gripper arm when playing a record. You shouldn't be able to feel any vibration.
If the grommets are at fault they are easy to replace. They are dirt cheap too, over here a set is less than a pound.
I've found the best method of changing them is to pull off the little circlips and washers from the three support pillars and prise out the old ones, whilst supporting the motor with your hand when removing the last circlip. Replace the grommets and feed the motor pillars back through the grommets and replace the washers and circlips (be careful not to drop the little rascals!)
A couple of drops "3 in 1" oil on the motor spindle and the jockey wheel axle won't do any harm.
If none of this is applicable and I think the symptoms you describe other than the spring suddenly becoming loose, would normally just build up over time, you may have a problem with the actual motor.
Can't offer you a manual for that model. But you could download one free for a 477 from here, the mechanics will be very similar.
http://thearcadeboneyard.com/RockolaJukeboxManuals.html