Could be servo, controller, or brake pedal switch (not the master cylinder automatic one).
First I’d verify the brake fluid level is at the proper amount, anything over, and this could trigger your problem, as well as under. Then I’d yank the speed control servo and visually inspect it. It could be slipping, or the motor could be failing for the same reason my own croaked. Which I actually just wrote about last night here(halfway down):
http://ericthecarguy.com/kunena/12-What-Are-You-Working-On/50288-1995-ford-f-150-xlt-302ci-5-0l-4wd
It is located against the driver’s side fender near the firewall inside the engine bay. You should be able to freely spin the drive motor without much effort from the larger connected plastic gear, and nothing that feels like it’s catching when you spin it.
If that doesn’t show anything significant I’d probably just replace the brake switch, well, just because it’s cheap and the next likely culprit, after that you’d need to do the FSM procedure for verifying certain resistances and voltages which I recall seeing in a couple of F-150 forum threads for this.
I’ve really never heard of one of these cruise control micro-controllers failing, it’s ALWAYS something else. As it’s intermittent, that makes it that much more unlikely. So you can just about rule out the main cruise control controller.