My '06 Monte 3.5 flex fuel rpm fluctuates during cruise at or about 65 mph. When things are going good I run about 1500/1600 rpm and on flat ground about 35mpg on the instant readout. When things are bad the rpm pops up and down between let's say 1600 and 1900rpm and the instant mpg drops to 30mpg at best.
This comes and goes intermittently but is becoming more frequent in the last month.
I almost wonder if the tach isn't going bad?
I'm not getting any engine light or failure code of any kind but I can feel a tiny tug and jerk as this happens. Honestly, it's such a tiny bit of motion that I had to start paying attention to the point of turning the radio and blower fan off to isolate it. I even doubt myself about this because what I think I feel is so tiny and my logical mind says a 300 pop in rpm should produce a more noticeable tug and jerk.
I did shift out of "O" and into "3" to make sure the trans was in overdrive and not hunting between overdrive and 3rd. There was a definite shift down to 3rd and back up to overdrive.
Now I wonder if the torque converter might have some lock up feature that is slipping? I have no idea really, just thinking out loud.
I work until midnight and I only notice this when I'm on my way home at night with no traffic on the road. There is so little traffic at that hour that I can monitor the rpm etc. vs. during the day traffic keeps me too occupied and speed varies too much to keep accurate track of what's going on.
The Monte has 153000 miles, starts right up and idles well. Original plugs, wires, ignition, fuel pump/injectors and trans fluid. Other items: coolant, oil, brakes, tires, front sway bar, tie rods, hoses, serpentine belt, exhaust are all replaced and in good condition.
I searched for a thread addressing this and failed to find one. If you know of a relevant thread or have any suggestions please reply.
Thanks for any help!

 

1. The 3.5 in my Aurora (yes, I know, different 3.5) acted up like this before it grenaded. I now have a pushrod engine. I will no longer have anything else. Technology is great, until it fails. That said,

2. I cannot imagine why you stlll have the original plugs and coils at 153K miles. Get thee to an auto parts store and change all that out before you even think about doing anything else.

 

96z34man
The simple answer to your second comment is I found no need to replace them yet. My Monte is my commuter. I bought it 2nd hand as a used rental car with 44k on the odometer and I've enjoyed it as a cheap, reliable, comfortable ride ever since. I really only do the necessary upkeep and reap the benefits of a decent, fairly economical, daily ride to work. This past summer I averaged 30 mpg per tank of gas which included all the highway and local driving combined.
*For fun and adventure I have a 99 Camaro SS in the garage.

 

After thinking about 96z34man's comment I tried to pull a plug yesterday just to take a look and see what kind of condition it was in. The plug wire would not come off with reasonable force! I gave up on the mission on the spot since I didn't want to tear the wire apart with excessive force and have my ride stuck in the middle of the driveway. I'll have to buy a set of wires and plugs if I want to take a look so it makes it a moot point as to the plug condition. I'll have to price the coil pack to make a decision. Remember I am not getting any codes, no misfires etc..
I'm still wondering if anyone who knows more about transmissions has any input to this fluctuating rpm problem. My gut tells me this might be a sign of the torque converter slipping somehow or some kind of imminent transmission failure.