1990 Mazda Protege is Sputtering

1990 Mazda Protege
134,000 miles

So, my car started sputtering one day after about 20 minutes of highway driving. It doesn't sputter at idle, or when the car is in over drive, but only when I have to give it gas to make an incline or pass another vehicle. The sputtering varies in strength and sometimes doesn't occur at all. About a month ago the sputtering became non-stop. After looking it over, and running it to check different items (like fuel supply, etc), the car refused to even start.

So, I took it to a nearby mechanic who said the distributer was bad. We replaced it. He also replaced three of the four spark plugs. One was stuck in the motor, but was firing okay, so he left it there to save labor expense. The car ran okay. Not what it used to be, however. Soon, I started to notice that the car seemed to get bogged down when climbing hills. A few days later, the car began sputtering at all speeds. I could hardly get it home at 30 mph.

I took it back to the mechanic. He said the plug wires were bad. They were replaced. I picked up the car and it ran great! For about one day. I hadn't driven it on the highway. Today was my first day with the car on the highway, and it started sputtering all over again after about 20 minutes of driving.

What could cause this such that replacing the distributer and wires would help for a short period of time? I'll hop back on and post results from my drive home. I don't know if the condition only occurs after the engine has reached optimal operating temperature, but I'll be sure to make note this time.

I've thought it could be that, after heating up, the Fuel Pump begins to fail, but I can't seem to find a mechanic that agrees with me!

Any ideas?

I appreciate any input!