My '87 does the stop instantly when turned off, the '89 runs really slow, growls, jerks and occasionally stops, works slowly even on high.
The '89 will be attacked first as it's 4x4 and snow is already in the forecast! I'll try a photo or two in the process.
I've stripped a bunch of F150's over the years to sell parts. I always check the wipers before stripping. I've seen rusted or "dry" linkage causing the jerks, the switch is usually at fault for the instant stops. Occasionally the wiper motor is bad, most issues I've found are bad or missing ground wires, as the motor sits in rubber mounts. Every intermittent controller I've found was good.