'91 300E sluggish

[p. rex]

I have a 1991 300E with the 3.0L M103 engine. I've been driving it for the better part of a year, and it has been painfully sluggish, especially below 3000 rpm. It hesitates badly off the line, and labors to maintain speed, even around town. I'd accepted that this was normal, but after having a mechanic replace the air filter, plastic intake hose, and fuel pressure regulator (which happened to be leaking), it ran brilliantly for just a few days. Completely smooth and competent, even quick.

Now, it's driving like it had been before, slow and unresponsive to throttle. What could possibly be causing the problem?

Incidentally, the problem seemed to return about when I filled up the tank. I would be inclined to think this is a coincidence, though, because I filled it up with Shell V-Power at the station I usually go to, and the gas I have bought there has not caused any problems with any of the other cars I drive. I have never put anything but name-brand premium gasoline in the car.

*Edit* I should probably add that I replaced spark plugs and wires not long ago. I don't know how old the cap and rotor are.