if its running better at higher RPM, then its prolly your primary injector that is going out. check to see if you are getting a good signal TO the injectors, do this by hooking a noid light to the plugs while the car is running. i know that you wont be able to start the car with the primary injector disconnected, so disconnect it while someone holds the throttle above 2500rpms, and then stick the noid light in.
if you cant afford a noid light, get one of those small christmas tree light bulbs, and bend the terminals out so you can stick them into the plug, it will light up just like the noid light will