Multiple cyl misfire DTC P0300

Drove the car to work last Thursday morning and everything was working great. When I went out after work it started but the engine was shaking like it had a misfire, and my check engine light was flashing. Rented a code reader from Auto parts store and I got a DTC of P0300 (Multiple Cyl misfie). Did a basic tune-up replaced plugs (were bad), replaced air and fuel filter (fuel filter was dirty), checked for vacuum leaks (none), checked PCV (clear).

After I did all of this I still had the really bad idle and a smell of gas in the exhaust. Next step to me was to Replace the Crankcase Position sensor and I noticed that there was no longer a smell of gas in the exhaust but the issue is still there. I have a lot of pressure out of my exhaust pipe, and I banged on the CAT to see if it sounded hollow or solid, sounded hollow.

I did notice that when reviving the engine to see if the issue still presisted at higher RPMs the engine dies at 4K RPM. Not sure if it's the CAT producing enough back pressure to kill the engine or if it is something else.

I used a higher grade of tester and noticed that while the engine is running the timing kept advancing and retarding many degrees at idle. I saw up to 13 degrees on the high end and 6 degrees on the low end. Is this normal? I have never heard of an engine trying to adjust it's timing so much at idle.