1990 Chevy Corsica afer driving a few miles, loos spark in

The exhaust system was getting glowing red hot, which means gas is being pumpted and burned in the cataliysit convertor. So I checked the spark. The car has three coil packs 2 pistons per pack. I found no spark in one set and checked to make sure that it was not the wire. One of the two wires had spark from that coil pack. So I installed a new coil pack. Car started up great but after 3 miles started to act up and only run on what seemed four cyclinders. . So I think that it may be in the compter system. What can I check? I did have a scan run by "schucks" and they found no error code problems.