I've been having the same issue lately and have been working to narrow it down. In my case shutting it off and restarting doesn't usually work, or it will work only if I let it sit until it cools. I have noticed this problem becomes more apparent with more passengers or load as well for whatever that's worth.

Given that I've replaced almost my entire fuel system over the last year I can rule that out as a possible source. I can also state that it isn't en exhaust issue since I've replaced the exh. manifold and my second turbo only has about 50k. I've adjusted the valve tappets, replaced the plugs, replaced all teh filters, and replaced the vacuum lines as well.

My thought is that it is a faulty knock sensor. I get a cherry turbo connector, but this can be normal under hard driving conditions so it's hard to take that as a sign the knock sensor is faulty. The only way I know to check is to pull the sensor and check it with a multimeter... if you get like 4.2 mOhms that is normal, if you get something ridiculously low the sensor is shot.

I've also though it might be the o2 sensor for some reason, but I'm planning on just replacing the knock sensor since the car is at 216k miles and I'm sure its near its life expectancy anyway.

Anyway, the "turn off to reset" symptom has led me to conclude that, whatever it is, it is electrical and most likely a faulty sensor. As bricks get older this problem can also become amplified by valve slap; it's possible a faulty knock sensor could detect slap as knock and retard the timing.

good luck,
rt