'98 Sil...shifts wierd

[mitunes75]

I'm nowhere near being a mechanic, so I'm sure everything I'm mentioning isn't going to make sense:

First of all, it has been noticed that once the gas gauge gets to about 1/4 tank, it will start shifting oddly. Starting from a full stop position, it'll go into phases where it practically won't go at all but you must keep pressing the gas and it'll finally start going (kinda like a freight train or semi), then it'll run fine. At this time the check engine light is on.

I'll fill it up, it'll stop, but everyone once in awhile the light will come on and sometimes it'll shift perfectly fine, and other times it won't. There are times though where I can run it one, two, three tanks and not have any issues with it.

Tonight was interesting. Engine light was on. Driving home from work. I noticed that it was idling real high and that it was shifting at weird spots, like 45 mph and 60 mph. I stopped at a light and it was idling pretty good (at the 2 mark). Then, I just let go of the brake and it was driving on it's own and ran at a steady 35 mph. When I got it home and put it in park, the rpm gauge shot up to 2.75 and sat there. I shut it down. Waiting a minute, cranked it on. It idled below 1 (as normal), and engine light off.

I just don't know what to do with this thing beside "take it to a mechanic". I'm just scare of what it could be or what it could cost. I was thinking it could be as bad a transmission replacement, and as easy as a sensor replacement.

My vehicle has 210,400 miles on it. It sat for two years before I bought it. So, I dunno if it just needs a fluid change or more.

Any ideas?