"it passed the test a couple of times but then it started to fail"
Could you elaborate on this a little? Where was the starter when it "started to fail"?
Did it test OK on the bench but fail to work on the rig?
Starters that work on the bench have every chance of not working at all when installed in the vehicle. The electricity cannot pass through a bad connection i.e. it will spin OK at 30 amperes of free-spin, then play possum when it tries to consume 200 amperes on the engine. Got it?
The PM (permanent magnet) starters are infamous for eventually wearing carbon brushes down until the point "They Almost Work". The brush copper leads are spot welded onto the steel brush mounting plate so the whole shebang comes as an assembly.
Auto Parts store rebuilt starters are woefully rebuilt. When I had to rebuild an auto parts store rebuilt starter 3/4'ers of the parts were tossed over my right shoulder into a dumpster.
Ford genuine parts (remanufactured) are rebuilt correctly. Ford isn't afraid to stick a brand new armature or drive end housing in a reman, whereas an el-cheapo rebuilder will patch, cobble, and pound parts to save money. Yes the Ford reman will cost more. I always weigh the savings of an El Cheapo versus having a premature el cheapo failure happen during a driving rainstorm.
My "money" is on your starter having worn-out brushes. They get stuck and carbon "goo" glues the brush to the steel brush holding clip.
