i had a little makita trim router the shaft is bend, it will make high pitching noise, and shake alot, try to put a big router bit, turn it on then off, and keep your eye on it, this way you might see if the shaft is bend or not, holp it help

If the router was ever dropped the base might have bent out of square with the spindle. Check by chucking in a 1/4" straight rod, lowering the chuck below base and check with a square. You could also use a dial indicator at this time and check for run out. Could also be excessive end play in the armature causing the bit to bounce up and down as it cuts. If the bearings were bad enough to cause bit chattering I would expect the tool would also be very noisy. Good luck!