348 Spider Slow Down Warning Light

Hi Guys. I know this topic has been dealt with quite a few times but I just want you guys to have a think about this problem and give me some advice.
My car was originally a UK car so when I bought it it had Cats. Over here cats are not required so I bought the cat replacement pipes from Superformance in the UK and fitted them. The car went fine for a few years then the problem started. While driving it would shut off one bank of the engine and the car would carry on, no revving, no power and flat sound. Some times a few seconds later it would go back to normal on its own or I would have to switch off the ignition and restart and all would be fine. But no warning lights came on. It could go months without a problem then the problem could come back once or a few times in a row. I took it to our Ferrari dealer and as what normally happens when you do this is it behaved perfectly no matter how much they drove it around and they couldn't find the problem and their equipment didn't pick up anything. They did find that the Slow Down warning light bulbs had previously been removed. They replaced them but the warnings didn't come on when they drove the car. They felt the problem was faulty temp coming from the exhaust so they took the thermocouples out the pipes strapped them away from the pipes, plugged the holes, told me to use the car keep an eye on warning lights. A while later the problem came back again, shut down one bank and slow down 5/8 warning light came on. I then switched the the thermocouple control units around to try work out if the thermocouple or the control unit was faulty. When I drove the car the other slow down 1/4 warning light came on but the car didn't shut one of the banks down. The warning light stayed on.
From all of this I now believe that my problem is a faulty control unit that is now on the 1/4 bank side. What do you guys advise? Am I on the right track.? I have seen the part is about $ 650 wow. Is there a cheaper alternate or fix?