95 Grand Prix starts occasionally, but will not stay running for more than 15 sec

My 95 GP (3.1 engine, w/ about 100k miles) will not start most of the time. Occasionally it will start but will only run for 15 sec or so. When the key is turned, the car makes a chugging noise like it is going to start but it will not turn over. It can be jumped but when the cables are removed it dies. I have had the battery and alternator tested recently and they are fine. Any suggestions as to what is could be?

Thanks

Check all battery cable connections, including the cables themselves, for dirt, corrosion, and grease. it sounds like a bad ground to me. Try hooking up just the black jumper cable from the battery to a good ground and see i the car stays running. Also try using the jumper cable from the engine block to a clean spot on the chassis, as you may have a bad engine ground strap.