Slow down lights flashing

I was driving last week on a 40 round round trip jaunt and halfway through my slow down light flickers on, then comes on permanently. So I stop the car, wait, then restart. This seems to have reset the slow down light, but just before I arrive home, it starts flashing at me.
I checked the maunal - apparently a flashing light indicates the cat temperature is too high, but a fully lit light means the engine will self test then shut down one bank for safety.
I've been scratching my head (with my indie dealer) on the reason for this. Most likely cause is the sports exhaust I had fitted a month ago (tho it's taken a month to create this problem?). The dealer suggests to eradicate the sports exhaust as an issue that I refit the standard box and then try it. He aslo says that his diagnostic eqipment suggests the temp is running high on both banks - which would suggest the exhaust.
Has anyone else had this issue with a new sports exhaust, and if so how did you deal with it? Mine's a 355 Spider 5.2, which apparently is more sensitive as it has more sensors fitted. Any help appreciated before a long and probably inconclusive game of trial and error. Oh, and my exhaust is a Larini - anyone had a similar experience with one?