if my 98 f150 with 4.6 v8 has the heater switch in the off position When it is Started I will have no heat no mater how hot the engine gets. BUT if I come to a complete stop and take off with a heavey petal I'll get heat then. also if I start the engine with the heat switch in any of the ON positions I'll get heat when the engine heats up. in the summer my A/C will quit if I take off a little to hard (I hear somthing close in the ductwork) It will return if I turn the A/C off wait a few seconds then back on. thanks for any help.

 

Check the vacuum line going into the firewall behind the battery for a crack. Very common problem.

 

Your blend door is broken and is flopping around as you start or stop the vehicle.

This isn't a vacuum problem.

Steve