M112 engine recurring misfires on middle cyls under hard acceleration

[Can't Know]

Hey, folks.

Newb to the PP forum but been on some others in the past. You guys seem to have a few more technically oriented members so I thought I'd roll this past you. I got a problem that I can't get to the bottom of and don't really want to drop lots of cash with a shop letting them guess at it. Plus I like DIY, not just because I like to save money.

I have a 2001 E320 with the V6 used in so many other models. Currently about 120,000 miles, all maintenance and repairs are current. Also replaced all the cats for the rattle deal but paid out of my pocket (!). For the most part I do my own repairs, not a fan of running to the shop for things.

I'm pretty mild in the way I drive, got the hotrodding out of the way when a kid. However, I do get on it every now and again, which is when the other problem crops up. I don't know if they are related, just these three problems.

The only symptoms I have are these. When the engine is fully warmed up, there is a very gentle vibration that can be felt at idle in gear. (No, I already changed the mounts). It can be felt when the belt is off and goes away if you simply step on the gas a tiny bit and raise the rpm a hundred or so. (I have a feeling it would be there with the engine cold, except that the rpms are up higher, so you can't feel it.)

But this is really the most bothersome problem. Under hard acceleration, as it gets up around 4,000 rpm, the CEL flashes and the ECU shuts down the middle cylinder on each bank. Well I assume it is shutting down those two because of the resulting rough running, stumbling idle (as though two cylinders were shut down, that clears after a restart) and the only codes that show up are P0302 and P0305 (and of course 0300). It doesn't have to be full throttle, but it has to be more than moderate acceleration.

Last, the mileage is not quite what it once was, I get about 28 on the highway and used to get 30-31.

I don't know if these three niggling things are related (I almost suspect they are but probably that's just wishful thinking). I have read some threads about a miss over 4,000 rpm, but that doesn't seem to be the same as this one.

Engine-related things I've done (mostly as regular maintenance, not trying to chase this): MAF (genuine Bosch, not the insert piece), plugs (iridium), wires (Bosch), KPS, cleaned the EGR tube and valve, breather hoses, motor and trans mounts, cats (all four), O2 sensors, air filter, fuel filter. I've run seafoam and techron through the tank (up to two cans/bottles per tank).

The only thing I sort of wonder about is carbon buildup in the combustion chamber because I don't drive it hard (and really can't now anyway) as there have been a few threads about that causing an idle vibration. But I can't think how that would cause the miss/cylinder shutdown on hard acceleration.

I haven't tried swapping coils or injectors, since it's always those two middle cylinders and it doesn't make sense both of those would act up at the same time and every time. I think about fuel delivery, but I don't really want to just throw an expensive fuel pump at it. There is plenty of pressure at the rail, I didn't measure it but it sprays out good.

Anyway, that's about it. Any useful suggestions are welcome, particularly since with this economy I may decide to start running booze or something and you can't get away if you can't floor it.

I appreciate you taking the time to read my tale, have a good long weekend, hope you have some good plans.