R53 hiccup under throttle

This problem hasn't really just cropped up, but it's irritating enough to inquire about it.

If I'm driving at a steady state in one of the higher gears, and then tip into the throttle the engine will react with a momentary blip in power delivery, like a misfire.  There's a "bucking" sensation due the acceleration/deceleration/acceleration.  

It does not happen repeatedly; it's as if I've surprised the engine by asking for more power, then it sorts itself out.

It never happens when running through the gears, and doesn't even happen all the time.  It's exactly the sort of thing that if I took it in to a shop to have it "looked at", the result would be that they couldn't get it to do what I was describing.

I've changed fuel brands back and forth - Chevron and Shell - used a fuel system cleaner additive, with no apparent effect.

Should I be looking at ignition rather than fuel? 

Check this out.  Yoyo is a known issue in early R53.  Fixed it on mine.

http://www.northamericanmotoring.com/forums/how-to/38268-drivetrain-operation-vacuum-gain-system-vgs.html

Had the "mother of all misfires" on I-90, the other day.

Check-engine and traction/brake lights came on, and car seemed to be running on three cylinders.

Limped to exit, parked, shut down, looked under hood for thrown belt, hole in block, etc., and found nothing amiss.

Restarted and car seemed to be running OK, brake light no longer on, but took it easy on drive home.

Trip to dealer resulted in recommendation of replace plugs.

Dealer wanted $330 to replace plugs.

Full set of plugs is $35 at O'Reilly's, so replaced them myself.

Slow degradation of plugs, resulting in occasional/random misfire. appears to have been the problem.