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Car bucks intermittantly

2003 Subaru Forester began bucking going up a hill, air conditioner on, very hot afternoon. It doesn’t quit running, but slows to 5-10 mph. The same scenario has happened 4 times in past 3 weeks. I have it towed to shop…all belts, filters, fuel sensor, spark plugs, valve cover gasket sets, oil, timing belt, have been changed. I drive it 50-100 miles perday for 3 or 4 days then it happens again.

Does anyone have any other ideas or has had the same problem?

Well, none of those items you mentioned likely have anything to do with the problem.
Making a fairly wild guess here with limited info, I would say that you may have a failing fuel pump. A pump can be a hit and miss affair; work fine for 6 days in a row and give up on day 7 followed by working fine again.

This could be especially true if the mileage is getting up there and the fuel filter has seldom if ever been changed.

You could drop by a local AutoZone, etc. and have them pull the codes. They will do this for you free and it only takes a few minutes. It’a always a good idea to know if the engine computer has any info on file but a fuel pump going through a few hiccups will not leave a code.

If the problem always occurs while going up a hill, this does indicate a fuel delivery problem. The fuel pump delivers the fuel to the engine. A load test of the fuel pump should/may reveal if the fuel pump is getting weak. The higher the load on the engine, the more fuel the fuel pump must pump.
A load test can be done with a fuel pressure gauge attached, the car being driven at highway speed (uphill, if possible), and with feet pressing on brake pedal and gas pedal, simultaneously.

If the electrical power to the fuel pump is being interrupted, by the engine computer or through the electrical power supply, that needs to be determined, also. I just invented a way to tell if electrical power is being interrupted to the fuel pump: clamp on a wire to the power supply wire to the fuel pump; run the wire to the dash; connect a LED light (from Radio Shack) to the wire; run the ground wire to a screw in the car’s body. As long as the fuel pump is receiving electrical power, the LED will glow!

I’m curious, how many shops/mechanics will say, “Oh, I/we can’t/won’t/don’t do that!”? (You know, “Not invented here!”).