Power loss

I have a 1995 K1500 with the 350TBI on a 5-speed. I have had this truck for over two years, bought used and impeccably maintained. It ran fine until about two weeks ago when it suddenly started idling rough and lost what feels like about 50% of its power in all rpm ranges. This happened while the engine was running and was sudden with no prior problems.

A year ago it failed emissions and I had to rebuild the exhaust including replacing the cat, which had been removed by the previous owner. I also had it put through a complete tune up, including a new distributor cap and plug wires. It appears to be firing on all cylinders but simply has nowhere near its normal power, idles rough, and about half the time will not stay running without stepping on the accelerator pedal.

I have since replaced the spark plugs, the idle air control valve, fuel injectors, MAP sensor, fuel filter, taken apart the fuel metering unit and cleaned it out (lots of varnish), and installed new gaskets in the throttle body and fuel metering unit. As most of these were extremely old and possibly original to the truck in some cases I have not had a problem doing all of this. However the problem remains.

Where else can I possibly look at this point?