speedometer problem

[kenneth]

1996 F150 super cab 4x4-4.9L-I-6 I had the ring & pinion changed from 3.08-1 to3.55-1 with a pos unit in rear. Prior to having this done my speedometer worked perfect now at about 55 or 60 mph it is very erratic jumping back and forth about 20 mph. I took it back to the garage where I had the work performed and they supposedly installed a new sensor and tone ring now it does the same thing at about 75 mph instead of 55. This is a terrible problem when in cruise control. Any help appreciated---------Thanks Bluetick