No power to 2001 Yukon slt

[novice101]

Had a problem starting, looked at battery and found the positive terminal was loose, didnt have an 8mm wrench so I took it down to a battery store for them to take a look at it. They replaced to two battery terminals 'cause I have stripped them while trying to tighten them, this did nothing. They checked the battery and alternator and both are good. I can jump start without a problem, but when I turn off the car, I try to restart and no power. They think it has something to do with the security system for when the car is turned off, the security light blinks while making the beeping sound. They also found something funny with the negative terminal. the voltage is fine until the negative terminal is put back on. the voltage goes from a 14 down to 5 volts. Any suggestions???