1997 E320 M104 flat at low RPMs, backfiring?

[gmercoleza]

OK, my original thread basically outlined what I perceived to be performance issues with my new-to-me 1997 E320 M104:

http://www.peachparts.com/shopforum/showthread.php?t=191618

On the way home from work today, things took a different turn. Rather than the upper RPMs seeming "choked" now it is the lower RPMs, specifically anything below 2500 to 3000. When I step on the gas, it just doesn't go anywhere. Accelerates very S - L - O - W - L - Y. But then once it gets into the neighborhood of 2500 to 3000, it comes alive. If I shift manually and keep the revs up, everything is great, at least until I come to a light and have to start from a standstill - then the game begins again. If I floor the gas when it's "choking" then I hear what sounds like a backfire, except it's from under the hood, presumably coming through the intake?

I have been driving this car for 2 weeks now and these problems just became apparent to me within the last couple of days; I didn't notice them before, so maybe they were just getting progressively worse.

* No codes
* No CEL
* Did a visual inspection of vacuum hoses, connectors, air intake/filter, etc. and all seem OK.
* coils, spark plugs, and all connectors/wires were replaced last year
* air and fuel filters are recent