1996 Chevy Impala P0200

Hi, first time user here. I have a 1996 Impala SS which has been modified with a T56 6-speed manual. It threw code P0200 on me yesterday while driving around 30 mph and shifting from 4th to 3rd (very light driving, nothing seemed to trigger the reason for throwing the code).

The car immediately ran like it was trying to get out of quicksand -- very rough, had to hit the gas to make it do much of anything, and would backfire close to 3000 RPM -- I was too scared to attempt to rev any higher. It limped home and is now parked in the garage.

Any ideas what's going on? Thank you in advance!

-- Evan