85 f150 5.8 h.o.

this truck cranks very slowly even when being jumped. i have replaced the starter selinoid and the selinoid to starter cable. had the starter tested and it checked out fine. it only has a 500 amp VW battery in it. my main concern tho is that it ocasionaly continues to crank even after you pull the "s" lead off the selinoid. is this common in a "low voltage/amperage" situation?:confused:

The bendix in the starter pulls it out after the engine starts. The starter is probably going bad.
Use a volt meter and check the voltage at the starter. You need at least 11 volts for the starter to work properly.
If you want it checked find a local rebuilder who uses origional equipment parts. I've had better luck with junk yard starters than some cheap parts store starters.