Warning light proplem, new twist?

I have a funny problem with slow down light on a Mondial t. It appears that the ECU is the problem but I'm not sure if its just that it is not operating properly or if there is something else.

I have the typical problem of a slow down light coming on with the car running at about 3,800 to 4,000 rpms. The car has recently passed California's new smog check two program with flying colors, so the Cat. Converters appear to be fine.

To trouble shoot the problem, I swapped the ECU's from side to side and the slow down warning light changed sides as well. So I replaced the offending ECU but the new one is starting to do the same thing. I have tried swapping sides again and sure enough the warning light swaps sides again.

Although there has been a new twist. If the slow down light comes on at 3,800 rpms as I am cruising in 5th on the freeway, I can sometimes get to go off by down shifting to 4th and driving faster, say at 4,500 rpms. Sometimes, I just ignore the light and within about 2 minuets it goes off and will not come on again.

I have replaced both of the oxygen sensors and checked all of the connections.

Is there something I am missing or are these ECUs that touchy?