Do you hear any valve pinging when you initially accelerate?

If there is any pre-ignition or detonation (valve ping), the computer will retard the timing--I think by as much as over 20 degrees--and this could cause a "bog" under load.

The pinging can be caused by carbon deposits in the combustion chamber (piston tops and valves) that pre-ignite. Detonation can be caused by a low fuel octane, clogged injectors, bad EGR (the EGR introduces relatively cool air into the engine), over-advanced ignition timing (not likely in your case), defective knock sensor, wrong or bad spark plugs, or fuel pressure under load is too low (check the fuel pressure regulator). I'm sure I left some out of this list.

I had a problem with my 400E where it would bog under a load, but I could hear the pre-ignition noise and I backed off, so I never really pushed it into the higher RPM range; but I know it would have really bogged down (or blown up).

The solution for me was to run 2 quarts of Chevron Techron with a full tank of gas (twice). I don't know if it was a clogged injector or a carbon deposit somewhere in the combustion system that was causing it to pre-ignite, but the Techron was like magic. I started to see a difference within 100 miles and after the second tank, it never recurred.

Maybe you've already tried this. If so, disregard. If not, it wouldn't hurt to try. I would recommend changing the engine oil after running 4 quarts of Techron through the engine, though.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles