1998 Sonata
 

I have a 1998 Sonata w/ an automatic transmission and 4 cylinder. I recently purchased the car from a man who it broke down on.
The drive plate was busted. I replaced it with a new plate from Hyundai.
300 miles after the repair the guy I sold it to calls me saying it's broke down on the road with the same problem.
I've heard the harmonic balancer can cause this to happen and the car does vibrate at idle but are there any other common occurences with this motor/trans combination that I should be aware of that would cause the same thing?