My car one afternoon wouldn't start

My car one afternoon wouldn't start. It had previously just been drivable, running and after a 20 minute stop and park the car then wouldn't start. I checked a few things which I thought may have been a problem. Firstly I tried a different battery to see if it was a battery fault. No solution. I then check the wiring for any loose cables. This did get the car going for very short periods of time (car drivable for 40 metres before losing power). I then thought it could have been an alternator problem. The alternator was taken into a mechanic shop and tested on a machine. Everything was fine with the Alternator. After putting the alternator back in the car, it's still just runnable for 40 to 50 metres before losing power. I can't figure out what it could be, but with this information above is there anything that can be worked out what the problem I'm having may be?