Check Engine Light 97 SL 600
 

I'm having a terrible problem with a check engine light that keeps coming on - multiple trips to the shop but its a never ending problem.

Fault Code PO300, PO304 #4 cylinder misfire codes and P1519 Right Camshaft Timing Solenoid.
Have recently had #4 ignition coil and plug connector replaced. Does anyone have any ideas what might be the problem....Its driving me crazy - ever couple hundred miles it costs me a couple hundred dollars....

Thanks