1995 Toyota Corolla 95 Corolla Runs Ruff, Missing under loa

My wife's '95 Corolla has frequently required tune-ups, but she's not told me she's been taking it to a professional muffler shop for these services. She says they replaced the distributor cap, spark plugs and wires and did a "tune up" for $165 last time.

Now I'm driving the car 76 miles round trip each day to work. The car has suddenly developed a loss of power last week when running on the highway. At level stretches, the motor appears to be running fine. But when climbing hills, especially from a standing start, there appears to be no power, no accelleration, and the car can barely make it up. Gas needle drops at a record rate, and I'm filling up twice as fast as "normal". At $3.39/gal, this is killing me!

I've changed the air filter, which appeared dirty but not clogged, and will consider the fuel filter, which has never been changed. The clerk asked me if I wanted platnum spark plugs when I asked for 4 new ones for my car, but I don't know the significance of that. And he couldn't explain it to me. $12 for 4 plugs seems like a lot. AND it didn't help the problem.

The car is 13 years old and I depend on it for daily commuting to work. What would you recommend? I need to keep it running if at all possible.

PS: I find if it's missing or running on only 2 or 3 spark plugs, if I back off on the gas pedal and let it coast at speed (not trying to accellerate) frequently the car will clear the problem and suddenly surge back to normal. This also seems to happen when the automatic transmission shifts gears as I accellerate.

But crossing a bridge over the Ohio River from a standing stop. The line of cars and horns behind me is embarassing! Please help me!