1996 Subaru Legacy Stalling at idle

My 96 subaru has been parked all winter in Maine, I went out to start it the other day and move it. It started just fine and I let it idle for at least 20 minutes before moving it a short distance. Later as it was running I opened the top of the air filter box to check the filter and the engine, which was running great, started to sputter and cough and eventually stalled. Now it wont even stay running. It will start and the high idle after start sounds great, but when it idles down it immediatly coughs and then stalls. If I hold the throttle it will rev up, but will then do the same thing. I have added gas to the tank ( it was near empty) and dry-gas and fuel injector cleaner. I also unhooked the negative cable on the battery for an hour or so to reset the computer, no change. I am thinking that either a small particle got sucked into the throttle body when I opened the box, there is a screen over the opening so nothing bigger than 1 mm would get thru, or the O2 sensor got messed up when I opened the box. I am leaning toward O2 sensor because if it was a plugged injector it wouldnt run smoothly at all, and it does run smoothly at times, but only for a few seconds. Any info you might have would be great.