The car

89 daytona
Intercooled 2.5 w/a555
Stock computer controlling boost
3 inch cat back exhaust

The problem

Started my car after work yesterday and took off for home. Slowing down for my first corner I noticed that my exhaust was not popping on deceleration as usual. Looking at my afr it was reading 10s to 12s when it is usually high 13s and 14s. Coming to a stop it stayed rich for a bit then levelled out. It seems to be acting normally under acceleration and boost. I reset my computer but it made no difference. If I put the car in neutral and roll up to a stop my afrs are normal, but if I engine brake it runs pig rich again. Any ideas what I should be looking at? My AFR is taking a lot longer to calibrate on startup now and I am afraid of ruining the sensor.