Failsafe Engine Mode with Cruise On
 

I got the failsafe engine mode indicator (for the first time) on my 1998 XJ8 L after a hard acceleration and assumed that it was the cause. Read a few threads and noted that the cruise control could not be activated while the failsafe indicator was on. Parked the car for a few hours and restarted it with the cruise control switch on and the indicator was off, but after about 15 minutes of city driving the failsafe indicator came on again. Let the car sit for about 2 hours again, and started it with the cruise control off, drove for about an hour and the failsafe indicator did not turn on. Is this an indication that the culprit is the brake switch?

Checking what the codes are might narrow down the cause. Post the codes from the engine management in here when you get it. Installing a new brake switch might not do anything other than making your wallet lighter.