97 F150 Vapor Lock Maybe?

[tnteverett]

Need some advice before commiting to the $2k-6k work of replacing heads or engine.
The truck was taken to the shop for two complaints; pinging and sound of water running under the dash.
The result was that the MAF was cleaned, the thermostat was replaced, and I was told there was a possible head gasket leak. The mechanic said he ran a test that should have changed the color of the coolant if there was a leak but the test was negative. The mechanic also said there was no evidence of ping, either audible or in an error code.
After the repair the ping was still present. After the repair the truck now overheats. When the temp guage pegs, the truck looses all power. It will idle forever but as soon as power is required the truck will not respond. If I let it cool, sometimes les than 5 min (whenever the temp guage comes back into normal range) power is restored and I can drive forever. This usually only happens once after starting from a cold state. After that the truck runs normal with only a slight pinging problem (my original complaint).
Any ideas?? Should I replace the heads ($2-3K)? Should I replace the engine ($5-6K)?