Hi all, I have a '73 F100, and its been popping out of gear when I drive it, but if I rev the motor a few times it will drop back into gear. I believe that it's only doing this in 1st, because it normally happens when the car comes to a stop. Any insight? Thanks in advance.

 

Triple check fluid level...
Any leaks?
C6?
Possible Modulator...

 

Like 75PKUP said, first check the fluid level, make sure it's at operating temp and the engine is idling.
If that's okay the next thing I would do is check the linkage adjustment. Check the linkage for wear, in particular the stud that connects the gear shift rod to the arm on the transmission, these tend to wear and the shifter could be in "D" but the transmission might only be "part way" in gear.
I don't think it's a modulator, that doesn't really sound like the symptoms of a bad modulator.

 

Thanks for the info, when the rain lets up a little I'll check it. I was actually pretty curious about the linkage adjustment, so the pictures are helpful. The tranny was rebuilt 10k miles ago then sat for three years, so I'm hoping there's nothing serious wrong with it.

 

The way I adjust the linkage is I put the gearshift lever in neutral then I get up under the truck and loosen the nut on the stud on the trans. arm (the threaded part of the stud in the picture) and manually put the trans. in neutral and tighten the stud. The rod that goes from the steering column to the trans has a long slot in it to allow adjustment.