Backfire and sputter problems

Just bought a tacoma a few days ago. Took it for a test drive, checked codes (o2 sensor was only code) and then bought the vehicle. Vehicle had been sitting 6 months. So I put some seafoam in the gas tank with some gas and drove it 30-40 miles home. On the last turn before I got home I accelerated and started stalling and backfiring. So I swapped the MAF, plugs and wires, cleaned throttle body, new fuel filter, new engine filter, got an oil change and filter. Still having problems. Mechanic said I have a leak in my head gasket but I have never heard of those kind of issues from a head gasket and I don’t think I have a head gasket leak. Any ideas?

 

Also cleaned the injectors

 

I would check fuel pressure first.

 

I was told they checked the fuel pressure and they said it was fine. Could it be the engine still processing the seafoam?

 

I would suggest doing a compression and leak down test. "Backfires" are basically the combustion of fuel outside of the combustion chamber. I.e. burnt valve or cracked head.