Rough start, tachometer fluctuation

[faythsoldier]

Hello all, I have a '06 Aveo with 111,400+ miles on it (a lot, I know).

It's run like a champ with minor repairs and routine maintenance until about 2 months ago. When I start it up after it's been sitting for a while (usually overnight) - and ESPECIALLY if the gas is low - the tachometer jumps from 0 to about 4000 until the car has "warmed up." Also, if I try to drive it before it's warmed up, I have to really play the clutch and mash the gas to get it to even get to 10 mph. Sometimes on the freeway when the gas is low, I get really bad gas mileage (usually 24 mpg when my usual is 32-35) and have very little power, it feels like there is a lot of resistance just trying to maintain 80 mph.

I've run fuel cleaner through it, replaced the battery and the air filter. None of that has worked. I also only try to put shell gasoline in it because it's better than the cheap stuff, less water, etc.

Any help would be appreciated. I'm taking it to get a diagnostic on Saturday (two days from now) but I would like some owner feedback before I start pouring money in to it. Thanks!!

[Daox]

Check engine light on? Any other symptoms?

[faythsoldier]

Check engine light on? Any other symptoms?

No, the check engine light is not on. Hasn't been at all this entire time. I listed all the symptoms of the problem.