Replaced ECU, now car won't stay running

I recently had my 1990 Legacy wagon 5spd AWD die while driving and leave me on the side of the road. Got it towed to the shop and they said it was a bad ECU and corroded battery cable ends. I replaced the battery terminals and the terminals at the ends of the cables, as well as adding an extra battery-to-engine ground wire. I also got a used ECU from a reputable used Subaru parts dealer. Just finished installing all the new stuff, and the car won't start. Well, it will try to start and sputter and cough, and every 10 tries or so it will actually fire and run for 10-20 seconds (sounds totally normal when it does this), then die again. Just stopped trying when I could here that the battery was getting low from all the cranking. Is this normal and the new computer just needs to dial itself in with my car, or is something else wrong? The shop seems right that the old ECU was gone because before replacing that the car woulnd't even sputter...
Any help or ideas would be great, Thanks. Jon

Hello Jon;
Could be that you are having relay troubles. The relays are located above interior fuse panel. There is a bracket that holds three relays, left is fuel pump, center is engine management (computer), and on the right is the horn. Just below that is the main relay. I would pull all these and test them. Also check the ignition switch. The best advice I can give, Steven.

OR...... it could be the shop has no idea what their doing and the ecu wasn't the culprit?........