sounds like a fuel issue maybe, fuel pump maybe getting ready to bite the dust. But im probably wrong. something like this is hard to figure out over the internet. Anymore symptoms? This could be a couple things.
just adding my two cents but i really dont trust those fuel additives such as injector cleaner and what not. the only thing i trust to put in my fuel system is marvels mystery oil. Thats just me though. I have seen a lady's fuel injectors get fired because they added that fuel injector crap to her car. Just from personal experiences i dont trust them it might turn out different for you though. Good luck and let us know if you need any thing else. Defintely get the yune up though it couldnt hurt to do all that was listed above. Its like a preventive mantaince.A general tune-up means replacing all filters (oil, fuel, air), topping off the fluids (oil, brake, power steering, coolant), adjusting the clutch, inspecting (replace) brake pads, and maybe running some cleaners (fuel injector cleaner, seafoam, radiator flush, etc). Obviously, some things can be done at the same time.
Every 60k miles or so, you want to replace spark plugs/wires, replace PCV valve, replace drivebelt (serpentine belt), check the brakes (rotors, calipers, & pads), inspect/replace coolant hoses, inspect suspension (lubricate joints if necessary).
For high mileage (100k +), the differential fluid should be changed, and you need to start watching for things that might fail (water pump, alternator, power-steering pump, fuel pump, A/C,etc).
^^That's all what you should ideally do... but not everyone can/wants to (I know I haven't done everything. :yelwacko. I'm just describing a tune-up -- I'm sure you don't have to do all that just to fix your problem! :shigrin Like "fasterthenev" said, you probably just have to adjust your clutch.
And don't ask me how to do that: I have an auto. :laugh: lol