Surging and shuddering Mustang! HELP ME!

I have a 2000 Mustang and for the past month, it shudders and surges while stopped at a light or stop sign or while driving uphill. I have already changed: air filter, mass air flow sensor, spark plugs, wires, and then I finally took it into Ford because it was still running poorly and they replaced the O2 sensors and battery. This is all getting very expensive and my problem is still not fixed. I have a loss of power when going from stop to go and when driving uphill. Just last night while driving uphill I had to stop for a stop sign and the car felt like it was about to stall and it sounded like it was about to turn off, but I quickly put some gas into it and it went. Ford told me that they think it might be my catalytic converter now, but this is the first time after 3 tests that have come up as that. Any suggestions? Thank you!