Don't just assume that the head gasket sealant completely fixed the problem either. It's been my experience that it is only a temporary band aid at best and more often than not it only partially seals gasket tears.

The correct way to fix a blown head gasket is to replace it. Also, don't rule out warped or even cracked heads...

 

If you said you're getting p0300 that's random misfire on multiple cylinders. And yes, it could throw that code because that's what could happen

 

best bet would be to buy or borrow a noid light for checking the injectors. I had one that the random misfire kept coming back. I turned out to be an injector wire issue. The injector was fine but the damn wires were chewed and barely making a connection.

 

I had the exact same issues with my truck. We ended up having to replace a head. The same day I took the truck out of the shop it was running much better but under extreme acceleration (like merging on the highway), the RPM's rise and the truck will not shift, the CEL starts blinking and stays on. I take it to the shop and they say it is coding as engine misfire. 6 month's with this issue and don't know what to do...