It will rev up to about 1000 RPM's and then it acts like it wants to stall

I have a 1994 Chevy Suburban with a 5.7 350 and it has about 170,000 miles.
Whenever I accelerate slowly or even just sitting there idling and lightly push on the accelerator it will rev up to about 1000 rpm's and then it acts like it wants to stall. It will drop down to about 500 then will rev right back up again without me letting go of the accelerator. It will do this over and over again.
I have changed the cap, rotor, wires, plugs (twice now), fuel filter, and I even replaced the converter after finding out that my old one was completely hollow on the inside. I even went as far as "rebuilding" the throttle body with new screens and gaskets.

That is the only problem. When I am at highway speeds or even normal driving everything works just fine.
Would you have any ideas? ?