Drives for 20 minutes from cold then shuts off

Hi! Just bought this van used. Lots of engine vibration. Check engine showed 4 weeks later, CAM alignment sensor code, pay for diagnostic showed CAM alignment synchronizer, had mechanic replace. No more check light but still lots of vibration and mechanic says need harmonic balancer $400.00 estimate. Pulled out of garage pedal to metal 4,500 RPM's. Turned around drove quarter mile sputtered to stop. Stalling loss of power, back to garage, brownish oil high on dipstick, mechanic ordered/installed Blue Devil head gasket sealer. Sat overnight. Engine will run from cold about 15 minutes then pinging increases until stalls out, sputters, hard to start. After 20 minutes cool down will run for 5 minutes. Drove 60 miles like this! Paid for oil change, different tech says "intake manifold/valve cover gaskets appear to be leaking". $125.00/hour diagnose no time estimate. Internet is free how 'about it? Low low budget so have to ask.