I have a 93 ford ranger it won't crank...why?

I was driving it home when it started to sputter and the gauges started acting crazy jumping all around. Then once I turned it off it wouldn't crank, just the clicking. I jumped it off and it cranked and ran fine. After letting it run for a while I disconnected the jumper cables, it started running rough again and the same jumping gauges. the battery and alternator both tested good at the local parts store. I'm at a loss... anyone had this problem before? Any help is appreciated!!!