Engine jerks with loss of speed

[BSOD2600]

I'm not really sure what the problem is, actually. This weekend I drove ~700 miles and only twice, about 15 min apart did the car miss or something. Basically, both times I was driving at highway speeds and there is a really big jerk / temporary loss of speed. The first time I was accelerating up a hill in 5th from 45-65mph. Second time, was just driving on level ground at 70 mph. This time when it happened, I noticed the rpms dropped down a lot.

I wasn't low on gas. The timing belt/pulleys were just done the previous week. No CEL's were thrown when these jerkings happened. The car still seems responsive and doesn't lag (that I can notice). The car gets 23-24 mpg for pure highway driving.

Last week, I noticed something similar when driving up a hill to school, although it wasn't nearly as noticeable so I just dismissed it.

I realize this is a really vague question, but any ideas what this might be?

[Manarius]

My car did that about a month or two before my MAF went out.

[BSOD2600]

Well guess I spoke too soon about this happening randomly. Today on the uphill onramp, it did its little jerk/missing thing twice. Guess I'll replace my bypass valve with a new one (existing is sort of hacked together) and clean my MAF with rubbing alcohol. Hopefully it goes away, 'cause I really don't want to spend $300 for a damn part.

[BSOD2600]

Not sure if I haven't been driving spiritedly enough lately or what, but this damn jerking/loss of power thing happened again. Was pretty much WOT, going up a hill around 60 mph.

Is this really a sign the MAF is going out, or something else?

[Legacy777]

Yeah....it could be a sign of the MAF failing. If it continues, look at trying to find a used MAF sensor to swap in to see if the problem continues.

[BSOD2600]

I called up Smart Service ( a really good subaru shop in Seattle) and described my problem over the phone. The tech said he doubted it would be the MAF, since if they're failing its typically all the time. He thought the car was missing really bad, possibly because of a spark plug wire or a spark plug arching against the block.

Guess I'll try checking those this weekend.

[BB]

I'd point my finger at the coil/wires. Its most likely to cause a problem at peak torque/boost.

If there is not problems at redline or when you back off the throttle, I'd bet its the coils.

BB

[BSOD2600]

I checked all spark plug wires resistance and they all were ~11K. I couldn't get the damn 3 pronged connector off the coil (don't want to break it and then really be SOL), but I measured the resistance anyways. Primary AND secondary were ~13K, which is not spec according to the Haynes manual. I'm guessing thats because I didn't unplug the coil from that 3 connector?

When reinserting the spark plug boot for #4, I noticed it doesn't really 'click' in like the others. Seems a little loose. Maybe thats the cause of the occasional miss?

[BSOD2600]

Noticed today that now in any gear under some boost it's doing the jerk/temporary loss of power. Would a loose spark plug boot be a possible cause of this?

[BSOD2600]

Replaced plugs and wires, which seems to have fixed the problem . Odd thing though is after starting it up after changing them, I had a CEL for the knock sensor. After unplugging the diagnostic wire, the CEL went away. Didn't return during running errands later in the day too.

[BSOD2600]

Well the new plugs/wires might have fixed the missing problem for daily driving, it seems I still have my underlaying issue.

WOT in 4th up a hill near my school, once again the car missed bad. What else should I check for this problem?