Cruise Issues

Hello. I am new to this forum and I hope someone can help me out. I have a 1995 pete 379 that was born with a 3406e, and have since swapped it out for a 6nz C-15. Ive been having all sorts of trouble with the cruise. If you set the cruise @ 65mph, It will slow down to 60 mph and lock in, and it wont let you bump it up. Same thing if I try to set it at 60mph, It drops down to 55mph. and, every once in a while, the truck will just pick up speed and take off, and not stop till you shut the cruise off. Is this an interference issue with the signals coming from the speed sensors?(which both have been changed). when i did the swap, I used caterpillars 40-70pin harness adaptor. every once in a while the pedal acts up too. Its like you took your foot off the pedal but you didint. throttle position sensor has been changed as well. any ideas?