2 things come to mind , the first is that you need some adjustment , I think your TC should come in at about 45-50 , but thats why its ajustable to fit your driving habbits. next is I've seen this symtom at the correct speed that was caused by the brake sw. on the peddle , on eveything els I've worked on the sw. was adjustable , this was not so I hot glude a small washer on the foot of the sw. , if this is your problum than you would notice that the cruse would shutoff also.