92 f-150 will not crank over

Battery is good, Starter is good. About 10 days ago, my truck would not crank over, it was clicking at the relay, I waited a few minutes and it started. This happened for a while, then it wouldn't work anymore.
A friend said to tap on the starter with a hammer and it would engage and it would start. I did this when I needed to.
Monday, I went out to start it and it wasn't even clicking at the relay. I put the truck in neutral and used a screwdriver to jump across the terminals on the relay. The truck started right up. So, this worked for a while.
Yesterday, when I started the truck by jumping the terminals at the relay, it cranked over, but with limited power. It did start. I drove to the store. When I tried to start it again, nothing. I tried jumping the terminal at the relay, nothing.
I had the starter tested at Autozone. Starter is good. I put the battery from the f-150 into my Toyota and it cranked very well.
I also tried to jump start the truck and it will not jump start.
From what I can tell, something was burning out.
Would an inline fuse burn out slowly?
Does this sound like an ignition switch problem?

The relay has a small wire when trying to start it should have 12volt. Otherwise you can have a bad relay.

My 92 just did this same thing this last weekend, here is what I did to fix it. My truck still has the factory battery cable that splits 2 ways from the battery post clamp. One cable goes down to the starter solenoid on the starter, the other goes to the solenoid on my fender. I checked voltage at the fender solenoid and had 12v. I checked voltage at the starter solenoid and had 9v and it dropped to .3v when trying to start. Even probing into this cable 1/2 inch from the battery gave me this same reading. I cut off the cable going down to the starter and replaced it with a new one, which I just bolted with the short cable at the fender solenoid. Also replaced the small wire that runs from the other terminal on the fender solenoid down to the starter solenoid just to be safe. After everything was hooked up I gave it a shot. Started right up.

Thank you for the response. I do not have a tester. I am either going to buy a tester or jumper cables. With the jumper cables I can go directly to the starter and see if bypassing the wires starts the truck. I can also start the truck with jumper cables.

Definitely buy the multimeter. I got mine at Wal-mart for $25 and its digital and will do lots of different things. It has been well worth the money, has saved me from hiring a mechanic many times