This sounds linkage related. The cruise control bypasses the pedal mechanism. But there's small dampening shock absorber next to injector pump linkage might be the culprit. Another suspect could be the pedal itself blocked by carpet fold. Just dont start messing with lengths of linkage arms, they're pretty much calibrated to proportional lengths. Your 240D oughta be able to do 80mph all day long.

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'80 300SD/ w116
'79 240D 4-spd
'71 750cc Guzzi

previously owned:

'83 240D 4-spd
'77 280SEL 4-spd
'74 280/8
'72 250/8
'65 220Sb 4-spd
'63 220Sb 4-spd
'63 190c 4-spd
'61 220Sb 4-spd
'60 190b 4-spd