cruise control not letting trans jump in and out of lock up
 

i have had a problem with my cruise control not working..... some times it works some times it doesnt so and recently the trans will kick in and out of lock up i have 4 12" subs in my truck and i noticed that when my subs would hit it would jump in and out of lock up every time from the vibrations so i disconnected the cruise control module and it now goes into lock up with no problem does any one know the reason why it would be doin this

its a 94 gmc 1500 XC 5.7ltr 2wd

Only thing I can think of is if the brake light switch is bad.