1983 SC euro 089 WUR exhaust backfires

[SkiVT]

Looking for some ideas on an exhaust backfire issue. Car starts, warms up, and runs very well overall. When I accelerate "smoothly" up to 4-5k rpm or so I have no issues. However, when I accelerate aggressively to that same rpm range, after slowing back down i get backfires through the exhaust. The difference or trigger point seems to be when the 089WUR triggers a much richer mixture on aggressive acceleration. My AFR gauge in the smooth acceleration example stays in the 13.8 range without any sudden drops and doesn't result in backfires after slowing down. In the aggressive example, it drops to the low 12s, which I believe is normal. However, the outcome then is the exhaust backfires after slowing down. To add to the strangeness, if I pull over and turn off the engine for 20 seconds, I can start it back up and drive away with no backfires (until I accelerate aggressively again). My assumption is unburnt fuel making it through. If that is correct, then the question becomes why?

The WUR and FD were replaced with correct models two years ago (thanks Tony). Rotor, cap, and Clewett wires are all less than 1 year old. Owned the car for 5 years and this issue is new this summer.

Thanks in advance for your thoughts