2000 Toyota Corolla Choke on start-up. What could it be?

Upon starting the car, it idles just fine. Once I put it in gear, the engine chokes, the revs go very low and car almost dies. In order to prevent this, I try to pump the gas but to no avail. While the car is choking, even pushing the pedal all the way down results in no response from the engine, no revving. This is only while the car is choking. After about 10 seconds of this, the car is fine in gear and driving around time is not a problem.

I have changed the spark plugs, taken out the fuel injectors and cleaned them, and ran injector-cleaning fluid through the engine but none of that has helped. What could be the problem? How could we narrow it down from whatever the possibilities are?

Thank you,
Deyan