Fuel injector

That can be lots of stuff so don't just go the not enough fuel route (but in that vein you also need to check the fuel pressure as it could also be a bad pump or pressure regulator). And if that is all good then it could also be an EFI control issue caused by anything from a bad sensor (one of a dozen or so) to a bad computer. You need a code scanner to check that out for sure, and even then it may not find it, but never hurts to scan anyway from time to time if your car is of the older variety like my 88 Escort GT and doesn't have a check engine light.

That said, your problems cause could be even simpler as that electronic stuff tends to last a long time so maybe it could be as simple as a spark issue (cap, rotor, wires, plugs, pickup coil, timing, TFI Module, coil). If that stuff is all old or never been checked then I'd say replace, clean or adjust as needed. And remember, even if stuff like wires or a coil or a spark plug look good, they may not be. Sometimes you just have to guess and replace them.

On top of that, it also could be an intake issue (air filter, throttle body, leaky intake hose or clogged intake). Wonl;t effect low power demands but when the egine needs air the most, this will ultimately choke it and result in hesitation and deficient performance. Sometimes you even get squirrles and rats stashing stuff in there and that can really limit airflow, but only when you really need it.

One last thing to consider are vacuum issues. Makes sure all the vaccuum hoses are good and snug and connected and if you hear like odd hissing or sucking noises, especuially near a vaccuum coinnection, time for a new hose or fitting or whatever. And remember, the entire intake sytem is all paret fo the vacuum system so make sure it has no leaks either at the connections of the hoses and pipes as well as the intake manifold gasket.

Also check the PCV valve and filter. If they are clogged then you can get excess pressure buildup in the crankcase which seems like it wouldn't hurt so much, but at high power demands, every little bit can make a difference.

I hope that all helps but your problem is one of the toughest to diagnose as it can be caused by so many darn things, many very minor, but often very hard to find. Most people get fancy and go fuel system and electronics first (though a new fuel filter never hurts) but often it is best to start with a good old tune up (new ignition system parts) first and then work out from there. Much cheaper starting point and a tune up is always a good idea anyway so it won't be money and time misspent where as sensors and scanners and computers can be very pricey and they are often not returnable either.