overheating when towing

What is your question?

When towing i hear the fan kick on,about 5 seconds later temp shoots from normal to overheating,LED display says check engine temp,about 10 seconds later temp drops as fast as it went up and fan shuts off,shows small signs of A/F tank popping off pressure,no A/F loss,no codes,cooling system never exceeds 15psi on pulls with guage attached,pressure never builds up unless in hard pull and dealer saying head gaskets but not sure and wants 3800 to try it