1995 Saab 900 Engine Stalls at Low RPM

My SAAB 900 will start and rev-up but than will quit. If I rev-up the engine past 2500 RPM it will run, but when it drops below 2500 RPM it quits. If I drive the car it will rev-up past 2500 to 4000 RPM and drive OK, but when I let up on the accelator peddal it will stall and will not run unless I push the accelator down halfway to get the engine back up to 3000 RPM. It has not response to throttle from 0-2500 RPM. What is the problem?

Thanks,

Rob