Silverado Surging 1400-1700 RPM - GOING NUTS!

[RTICWOLF]

Hey there everyone!

I have a 99' Silverado 4x4 Ext Cab with the 4.8L engine and auto trans. I bought the truck 1.5 years ago at 59k miles and it now has 89k miles. Since I have owned the truck, there is a "surging" effect when I am at a cruise speed and the trans is in OD. It only happens when there is a load on the engine (such as going up a slight incline) or just even on a normal cruise. It is worse at highway speeds, but I have noticed it just fine at 40 MPH too. The tach will jump between 1400 and 1700 RPM and you can hear and feel the surge from the engine. I have learned how to drive it so that the effect is minimal, but its now getting much worse.
I had read here that it might be an issue with the trans and I actually had the ECM flashed with all the updates, inlcuding the trans update, and it actually went away for a while, or at least it seemed to. I also changed the fluid completely at that time. That was at about 78k. Since then it has came back so just last week I decided to have the trans flushed again. I also changed the spark plugs to Bosch +4's, new fuel filter, new PCV Valve, cleaned K&N filter, changed oil (as normal at 3k miles), and used Seafoam in the gas tank (did the intake clean at 65k with Seafoam down the brake booster hose already). What else could this be? Is it still a trans issue or perhaps is there something else? Also, within that past couple of weeks I have had hard starting issues if I shut off the truck and come back to it in a short time (an hour or less)....even has started and died right away a couple of times. So, my next thought was perhaps a throttle body issue? I have also read about issues with those, but I am just frustrated as heck at the moment. Do the TB's from the LS1 work on our trucks? Perhaps a new one is in order, but I'd hate to buy a new one and have the same issue with the surging. I love my truck, just am tired of all these issues (like everyone else...the clunk in the driveshaft...etc).
Any ideas would be greatly appreciated. Sorry this is so long, wanted to give the history in hopes that helps figure this out. Thanks much!

[stantonl]

Have you tried cleaning the throttle body that would probably help out some. Also you should try some fuel system cleaning.