Surging cruise control

I finally got to using the cruise control on my 1990, works well most of the time but once and a while when I set the initial cruise speed it'll dump the throttle and the RPM's will spike up 800rpm or so then settle back to normal. I also found that when setting the accel. it always adjusts the speed 5km/h higher (if I set it to run at 100, it will bump up to 105 and stay there).
Besides this, the cruise works great but I would like to cure the rpm surge, since it's likely guzzling fuel when that happens.

the RPM spike sounds like the torque converter unlocking to allow the vehicle to get up to speed, then re-locking. (my '97 will pull the pedal down a bit whenever i initially set the CC, then settle down)

by "setting the accel." do you mean using the ACCEL button to increase your speed? mine seems to go 2 mph per click, so your 5 km/h would be right on target.

Under the dash is the cruise control module. There are 3 adjustments on it see if one of them will fix it.