If I recall, there are two switches - one for brake lights, one for Cruise interrupt (called the Brake Cancel Switch)... have you checked or replaced both?

If I also recall correctly, if they don't report agreement(both open or both closed at same time) to the processor when the cruise is turned on, failsafe mode gets tripped.

 

I have replaced the white brake switch, but am unfamiliar with the other switch. do you know what it looks like, or where it is, or what the part number would be? As it turns out, sometimes the problem does not start for a few seconds after the cruise control switch is turned on.

 

@johnleavitt,
If you replaced brake switch, you also replaced the Cruise Interrupt switch QM is talking about. It's part of the brake switch. There are two micro-switches in the brake switch housing...

 

my 99 xjr did the same thing it end up being a broken t fitting on the vacuum line. mine was on the cruise control under the plastic on the driver side i had no codes so look for that stuff. it would have to be little or u would get a code hope it helps