Mine just started doing this last week. As you stated, mine works fine when moving as well. My C-1500 does this from time to time. Check the freon level. If good check the accumulator sensor. Not sure on the Fords but located on the accumulator body on the Chevy (when mine goes bad it will either not kick on the compressor, or cause the compressor to jump on and off constantly, sounding like a thumping noise). If that checks good, check the cooling fan(s) next. R- 134 does not transfer heat as effectively at lower speeds and requires an auxiliary fan to move air accross the condensor. Another possiblility is the TXV may have ice build up on the evaporator side (which happens a lot in older home cooling systems) which requires nothing more than shutting the system down for a few minutes to allow ice melting. That may indicate moisture in the system.
If everything checks out to this point you have to look at a deeper cause. Start by having a reputible shop check all the connections, looking fior anything wrong. Check all the electronics, such as the in dash controls. Finally, and most expensive, is the compressor. It may very well be worn out.
