1992 Lexus LS 400 Repair help

Electrical problem
1992 Lexus LS 400 V8 Wheel Drive Type unknown Automatic 140000 miles

I changed the spark plug wires the car now purrs like a cat and will not stall or shut off while driving but as soon as I shut it off it doesn't start when something get hot, when it cools down cranks right up. I took it to auto zone and they tested my amps in the battery and altenator which was kicking out as it suppose to but he noticed that the ampage was jumping back and forth non stop something he never saw before he said it could be a bad ground somewhere, I then had to turn the car off and restart it for him to test the starter the car wouldn't restart, just draining the juice out of the battery he said based on the numbers showing while trying to restart it that the Starter itself was bad. I've heard before if a starter solenoid was bad it would click, lights blinking & flashing. But my car turns the engine over it just doesn't fire up to start so I'm thinking my starter is fine. That will be a costly job I hear so I'm trying to avoid that if neccasary. Someone mentioned a possible bad main relay what do you think?