Mitsubishi Repair: CEL + Code P0170 = Guzzling Gas?, premium fuel, funny feeling

Question
Hi Linda!
Ok - here's the info before the story:
1999 Mitsu Eclipse GST (Turbo)
Automatic transmission
136k miles
On the dash it says "Premium Fuel Only" but I mean, c'mon. I use 87.

You'll roll your eyes when you read this, but my CEL has been on for about a year now. I've been getting oil changes every 3k miles, as prescribed, and the check engine light has never been a problem, other than that it makes me a little uncomfortable. I had my beloved mechanic (honest! prompt!) run a check on the car and he gave me the code that is causing the light to come on = P0170 Fuel Trim bank 1. They said that they can't interpret the code there and should take it to a Mitsu dealership for service but I am not inclined to do that for a number of reasons - (far away, expensive, I love my local mechanic and would like to give them the business if they are able to do the repair).
I did some internet work on that code and what came up was information about the fuel system. Fine. Ok.
It wasn't really a problem at all for ages but now things are starting to act a little wonky when I drive...

The new problems:
So, before I took it for it's last oil change (1/5/09) at 136k, I started noticing this funny "feeling" when I would accelerate on the road. Though this is an automatic, I drove stick for years (loved my Galant!) and it feels like something isn't "sticking" between 1st and 2nd or 2nd and 3rd. You know when you shift and the car seems to slow for a millisecond and the tachometer drops until you stick it in the new gear? That's sort of what happens. I had my guy look at the transmission when I last took her in and they said everything looks ok...
So that's the first part. The other part, which I fear/hope is related...is the mileage I've been getting as of late. ABYSMAL.
Ordinarily, as you know, the mileage on this guy is not all that great. The turbo really takes its toll. However, recently this thing has been sucking down gas faster than a fat kid with a milkshake. I'm getting about 200+/tank city or highway. Yikes! I'm thinking it's got something to do with that P0170 code.

So, the question:
what do you suppose is wrong here and am I going to have to take it to a Mitsu retailer or can I have my local guy take care of the work? Am I making things worse by waiting?

THANKS so much and sorry for the long-windedness!


Answer
Kate: I have a book on codes but someone borrowed it has not returned the book. I have seen that code before but for the life of me can not pull it out of my head. Still I believe your shop guy could handle it just fine, I bet he even has the same code book that I have. Only he has it. You could however if you have a AutoZone close by get them to code the car for you, they would do this for free and they might know the code. The problem is the computer found a problem in the system and so kicked on the light. By letting that go, the problem grows in to more things as the sensor to be doing the work gets weak and stops working all together. So the computer gets to the point where it does not pick up that sensor anymore so it thinks everything is fine. But it is not so your car starts to run worse and worse. Did you know the computer controls everything in your car, I mean even how it shifts? I think it is nuts myself but that is what has happen to our newer cars. With out the sensor doing its job the computer is not doing the right job. I think you need to get after your local guy and tell him to fix the problem. Yes you are making things worse by waiting, because it will just keep getting worse. I wish I could help you with the code but just one of those things. Sorry about that. Not to worry about the long-windedness I wish more people were like you that way. Do hope you get this fixed and your car is zipping down those roads in no time. God bless: Linda