My 94 F150 L6 runs good in initial start up but if I go to the store then get back in it will misfire. My mechanic got it to do it once and told me it was misfiring on 5the cylinders. He had a week and of course it never did it again. Anybody else have the same problem? Any ideas?

 

You'd be far better off (financially and time wise) trying to resolve this yourself.

Thing I'd do first.

1. Check for ANY stored or operating trouble codes.
2. Clean off ALL of your ground points till shiny and clean then coat with your favorite di-electric style grease.
3. Remove the PCM and examine it for leaking capacitors/burnt components. Your truck is at the age where this will very soon have to be addressed. Better now than far away from home.
Here's what you'd be looking for :A9x ECM's (and same years ECM's) Failures Due to Age
I don't see this as being the infamous EGR stumble as it seems to be random in nature.
My bet is failing capacitors in the PCM.

Bob