Engine Surge

I have a 00 Silverado Z71 Ext. cab, 4.8 motor and auto trans. 264,000 miles. I noticed recently when driving down the highway at times the engine wil begin to surge. Usually between 40 and 50 MPH. if i let off the throttle it will smooth out or if I give it more throttle it will smooth out. Any ideas what would cause this??

Do a full scan, sluggish o2 sensor is one possibility.

I have a 00 Silverado Z71 Ext. cab, 4.8 motor and auto trans. 264,000 miles. I noticed recently when driving down the highway at times the engine will begin to surge. Usually between 40 and 50 MPH. if i let off the throttle it will smooth out or if I give it more throttle it will smooth out. Any ideas what would cause this??

Most all times surging is the fuel pressure low insufficient fuel.

check pressure 52-62pSI.. replace fuel filter . then use the techron injector cleaner.
make sure you have 1/3 tank of fuel at all times or more. if you operate with less than 1/3 often then the pump is most likely failing.