Loss of power and fuel mileage. on 1997 BMW 318i

My 97 318i (192,000 miles) is losing power and mileage is very low, it started gradually but is getting worse and is now almost to the point of it not driveable. It seems to have decent power at low RPMs but has less and less power as the RPMs increase. I've replaced the fuel filter, spark plugs and air filter but that made no difference. The car starts normally and idles fine. I'm beginning to think it may be the catalytic converter.

Asked by for the 1997 BMW 318i
Yes, it could very well be a defective Catalytic Converter. If you have a Check Engine Light or Service Engine Soon Light on, then you may have a code P0420 -> which indicates a defective Catalytic Converter.

I would also do a fuel pressure test, just to make sure that the fuel pressure is adequate. Also, if you have other codes, then by all means trace those down as well.
Catalytic converter was the problem, ordered bolt on OEM replacement and power is fully restored.
2 more answers
idle control valve and pcv valve replace.
it is definitley the cat converter had to replace mine and now it runs like the day Ibought it.

P.S. hope you didn't sell it.