1994 Saturn SL2 Intermittent Miss

I have a 1994 Saturn SL2 with 155.000 miles and an automatic transmission that recently started missing intermittently. It's been getting worse. To me, the symptoms are a bit unique. Basically, the car misses under load, accelerating or going up hills. (Which is, of course, when it's most annoying.) Anyway, it also misses idling in Drive at stop lights, but it doesn't miss if I put it in neutral. (ie, the engine runs smooth in neutral.)
If the car was carburetored I will think this was a vacuum leak, but I'm not sure with a fuel injection car, I'm new to them.
I also recently changed the spark plugs, so I don't think it's them. (The miss started well after I changed them though, so I don't think I screwed putting them in up either.) The spark wires also look good, but I haven't tested them. Does anyone know what my problem could be?

Thanks
Jim