You were correct. I have adjusted my thermo element and the revving "up and down" has stopped. Nothing but happiness. After searching for further info I found the following post on another site which describes the problem and fix.
"Surging Idle Fixed"
I have 2001 Frontier 2WD with the 2.4L engine. About a year and a half ago, I started having idle issues and went through about everything I could find to correct it. Over the course of several months, I tried new vacuum lines, IACV, PCV, plugs, wires, thermal plunger, and MAF sensor. While the problem got a little better, I eventually took it to the dealer to figure out what exactly was wrong. I was told a thermal switch in the egr system was bad, the intake manifold needed to be removed and cleaned, and even though they knew I had replaced the MAF sensor once, they also suspected that the one I installed might have problems. Anyway, they call me 3 days later and tell me it is ready to go. When I arrive, they had it warmed up and ready to go. The next morning, when its cold, I am surprised to see the engine idle to nearly 2200rpm initially. When I drive it, the idle surges for about the first 3 miles of driving, but then it seemed to settle down and run good. I did notice that even after it warmed up, the idle was about 1000rpm. Doing a search on this forum and several others, that seemed to be a common complaint with many Frontiers. I figured I would give it some time, before calling the dealer that worked on it. I was really hoping I could avoid this, since it such a hassle for my wife when I do not have my truck and I have to use her car. I looked everything over when it was warmed up, looking for anything obvious that could cause a high idle. Then I decided to look at it while it was cold and noticed something interesting, the fast idle cam has two marks where an arm connected to the TPS should be when the engine is hot or cold. I had looked at this when the engine was warm and the "hot" mark was aligned spot on. With a cold engine however, the "cold" mark was about 1/8" from the roller on the TPS arm. Figuring I could always move it back, I adjusted the fast idle cam "cold" mark to be aligned with the roller on the TPS arm. I started it up, the engine idled to 1800rpm, dropped to about 1200rpm and after warming up for several minutes, dropped to about 700rpm. And on top of all that, the surging went away. Of course the fast idle cam "hot" mark is now slightly off, but the truck is now running like new.