Starter? You tell me please?

I have a 1995 Ford contour with the 2.0L engine manual trans. When we first got it from a friend it would not charge the battery. Changed the altenator, still didn't work bought new battery, no go. Finally in despairation I took it to the shop and they replaced the mega fuse at the back of the engine. So it was good for a while then it started having issues with the starter. We would just get a click when we tried to start not alot of clicks just one when I try to turn on the car then nothing, sometimes it would start. Changed the starter, good for a while now its doing it again. When we take the starter off and have it tested they tell us it tests good. When we put it back on sometimes will work for a while and sometimes wont. I am at my wits end, please give me some suggestions of what could be wrong. Its driving me ( or not driving me :( hehe! ) Crazy!
Oh yeah if I push start it runs great. WEIRD HUH?