No power above 4000 rpm?

[Legoinsaniac]

My '92 400E w/ 220,000 miles has developed a problem over the last couple months. Normal driving is unchanged. But if I get to about 4000 rpm, acceleration diminishes and the car just won't go any faster. Once the transmission shifts to the next gear, and gets below that 4000 rpm, the car will accelerate again. It's like the car starts going up an imaginary steep hill once it hits those higher rpms. Any thoughts? I've noticed a reduction in gas mileage from about 22 combined to about 18 combined over the same time period. I would imagine it's related.

The engine isn't missing and there are no unusual sounds.

Thanks in advance!