My name is ***** ***** i will be working with you and hopefully find your issue .
First as my fellow expert said you will need a dealer level scan tool to check for malfunction codes, which will not be able to read with a generic code reader ,which only gets "P0" codes we need to check for "P1" codes .
Most common is brake switch as he said and which you checked and replaced .
You need to check for codes as in your case i'm guessing you have a P1595 speed control solenoid circuit ..
intermittent hence coming on and off.
But this you will know after checking for codes .
First you could check the vacuum hose disconnect at the servo unit and check for vacuum, should have a min
of 10 In,Hg.. normally 15-18
Switch off engine vacuum should hold for 10in,hg.
But ones you have the codes you will know what your problem is .
Please get back to me with the codes if you have any or not.
Kind regards Greg..