Idle valve will not cause the engine to miss at speed, only causes idle speed irregularities at idle, and can cause a stall if it sticks during hard braking.
If you get surges and thumping rough idle, check the current through the valve (you will need a meter that reads 1A DC) -- if you get intermittant low (300 mA or so) "twitches" the OVP is shot and needs replacement or repair -- current isn't being controlled by the speed control circuit, but by bad power supply via the OVP.
Intermittant loss of power under load CAN be the OVP, though -- low current will cause the EHA to vary the mixture improperly. This will be worse if the fuel distributor is set incorrectly.
I'm going to have to check the 300TE, I suspect my rough idle and performance problems are from the OVP -- it gets very hot in operation, and it's the original 1 fuse type.
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!