01 bonnie cutting out

My 01 Bonneville se with 3800 V6 overheated last week. Was low on water and person driving thought adding water to overflow tank was filling radiator and drove it @75 miles low on coolant til it started cutting out. After this happened I pulled codes 0420 & 0440 (twice each) from OBDII. Found a brittle fitting on end of vacuum tube going to purge valve had been broken when I was pulling thermostat. Replaced U shaped fitting with soft vac hose temporarily. This tube collapsed as soon as vacuum pulled so it shifted even more erratically and would cut out at @2200 RPM in 3rd gear (I think). Acted like there was a rev limiter @3000 RPM. Replaced tube assy from throttle to purge valve. Still have same codes and car is still acting like it did with collapsed vac hose. Low power (if I floor it will not go over 3000 RPM and doesn't accelerate much like its not shifting into passing gear), cuts out, doesn't shift correctly (have to let off throttle to shift usually), 'rev limit' @3000 RPM (but only while driving, in park it will rev to @3800). If I hold it at 3000RPM while moving for very long a couple times the SES lite starts flashing for a minute. Engine sounds fine while parked, idling or revving. Trans fluid good. When shifting from 2nd to 3rd it cuts out and, with window down, I can hear it faintly sucking or blowing somewhere under the hood in rhythm with the cutting out. Anybody have an idea what may be wrong? Just lost job so cannot afford to go to garage, or be without car. Thanks

Welcome to AF. Sounds like a bad cat to me, for sure.

Think I read that's a pretty pricey fix. Any temporary work around?