My car has always gotten pretty good gas mileage since I bought it (stop and go traffic, occasionally rev up to 6.5k) but recently my car has been getting the worst gas mileage and I get hesitation every time I shift from first to second and sometimes the other gears.

 

It could be injectors, but start with the basics like fuel pressure. I once had this problem and it was simply the fuel pump o-ring wasn't sealing well on the fuel line.

Dave

 

where can I find replacement o rings?

 

just go to a part store with your old ones and they will have a pack of em for like 3 bucks. try doing a tune up to that should help sounds like plugs or wires to me.