check all the vacuum lines for leaks. the manual says it should have atleast 10 inhg.
or it could be this.
OVERSHOOT/UNDERSHOOT FOLLOWING
SPEED CONTROL SET
If the operator repeatedly presses and releases the
set button with their foot off of the accelerator (a “lift
foot set” to begin speed control operation), the vehicle
may accelerate and exceed the desired set speed by
up to 5 MPH (8 km/h) and then decelerate to less
than the desired set speed before finally achieving
the desired set speed.
The Speed Control has an adaptive strategy that
compensates for vehicle-to-vehicle variations in speed
control cable lengths. When the speed control is set
with the vehicle operators foot off of the accelerator
pedal, the speed control thinks there is excessive
speed control cable slack and adapts. If the lift foot
sets are continually used, the speed control overshoot/undershoot condition will develop.
To “unlearn” the overshoot/undershoot condition,
the vehicle operator has to press and release the set
button while maintaining the desired set speed with
the accelerator pedal (not decelerating or accelerating), and then turn the cruise control switch to the
OFF position (or press the CANCEL button if
equipped) after waiting 10 seconds. This procedure
must be performed approximately 10–15 times to
completely unlearn the overshoot/undershoot condition