86 740 GLE, Unsure of miles.

It basically has NO power at all. I have to give it a fair amount of peddle before I can get it to move, And It won't downshift at all when I need it to, Most of the time I shift it manually to get what power I can out of the engine. When the A/C compressor kicks in, I can actually feel is drag the car down slightly.

What would be causing this?

 

Power Loss happened suddenly: fuel injection problem or ignition system

Top speed is less than what it used to: clogged cat

Weak compression: head gasket possibly
do a compression test first? How is the alternator? Battery?
one of the fuel pumps bad or clogged fuel filter possibly