Dakota Cruise/ EZ-TCU surge problem

[speedwrench44]

67 C10 Suburban, L87 engine, 4L60E trans with TCI EZ-TCU controller. All working good. Installed per instructions Dakota Digital CRTS-2000 Cruise Control, (DD recommendation) speed input from EZ-TCU. Works but surges bad. 60 mph slowly drops off to about 55, then surges, unlocks converter and goes to about 62, then slowly back down again. About 30-40 sec to complete a surge cycle. DD tech told me a DD SGI 100-BT Speedometer interface may fix the problem. When I questioned him before I spent another $100+ on a may fix he got huffy and told me he didn't say may. That's where I left it. I tried changing the speedometer PPM in the EZ output from 4000 PPM to 2000PPM , helped a bunch but still surges about 3-4 mph. Tried it at 1000, CC would not latch up. Anyone have experience with this problem?