Several possibilities here. Since you have performance problems at speed, I would first check the fuel filter -- if the filter is plugged you will have pressure and delivery problems -- eventually it will fail to start.
First thing to check after the fuel filter is, alas, fuel pressure, and unless you have the proper guage, I'd have this done.
Next, check for the condition of the idle control valve hoses -- if they are hard and loose, you have a massive vacuum leak, and since the air isn't being metered, no fuel.
Your fuel distributor may well have gone bad, too -- they tend to corrode internally and no longer maintain the correct pressure differential, hence no fuel delivery.
Other things to check are the EHA (or frequence valve, depending on which you have), plugged return line, etc.
A bad overvoltage protection relay will give you trouble, but I'm not sure if this is a problem with your model or not, and it usually won't limit power at speed, anyway.
Check that all the wiring is in good shape -- loose connectors cause all sorts of trouble.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!