1992 gmc sierra k1500 sle stepside 5.7l 350 v8 idling issue.

When I got my truck it started fine but was
reving high then low constantly back and
forth. I have replaced the distributor and all
its parts, new sparkplugs and wires, new
starter, new battery and cables, new pcv
valve, new front end fuel lines and fuel filter,
new IAC valve, refurbished throttle body,
new fuel pressure regulator,  cleaned the
carbon canister and check for air leaks,
trans fluid and filter were changed Aswell
as oil and filter and air filter...... Now when I
go to start the truck I need to give it has
then in park it idles perfect. Then after a
few minutes of driving when coming to a
stop or even just slowing down in speed it
starts reving down to the point where I have
to give it gas while my foot is on the brake.
But while driving it runs fine over 1000rpm
anything less than it starts idling down.