2004 Chevrolet Silverado Engine idle 5.3

Engine idle on my Silverado drops below 5k when stopped at stop & go lights to the extent it nearly quits running. It has quit once and often the rpm is so low it affects the power steering at the low idle. I know it used to idle more at 6k (or higher) and would not fluctuate at stop/go lights.

I purchased this 2004 Silverado Z71, 4 wheel drive, new in October, 2003, so I know all of it's life history. About one year ago, I notice a decline in the rpm at stop/go lights. Acceleration has always been and still is very good. I assumed at that time I got bad fuel at the pump and did nothing, believing it would get better once the bad fuel was used. However, it didn't get better so I took it to my regular mechanic who did a complete throttle body clean. To know avail. He said it couldn't be the fuel filter or fuel pump or I would experience other problems.

I then took it to the local Chevy dealer who said nothing was wrong. The dealer said normal idle speed for my truck is 5k rpm and of course, when I took it to the dealer to have it checked, it maintained the 5k rpm.

My question is what can be the cause for such low idle? It has the fly-by-wire system. The Throttle Body and TPS are all one unit. My understanding is these two together control idle speed. Since cleaning the Throttle Body did not correct the problem, do I need to start replacing these items along with the Mass Air Flow Sensor until the problem is resolved?