92 suburban. surge at highway speed

[renville]

I've got a problem I'm trying to track down in my 1992 2500 suburban. 350 motor with 200k miles. runs and starts fine but at highway speeds the idle jumps up 200 RPM then drops back down then jumps up again... this happens in both overdrive and regular drive. I have noticed that this problem only occurs when i am just barely on the throttle. If I push the throttle half way to the floor it goes away at the higher speeds. This lead me to check the TPS. I get .53Volts at fully closed and a nice smooth transistion to 4.3V at full throttle. Book says .6 to 4.5V is normal. I did not notice and jumpyness from the TPS as it moved to WOT. Also the problem does not surface until the vehicle is warm. No trouble codes stored in the computer either.

thanks in advance.