Could a malfunctioning O2 sensor cause a 3x HC problem? Don't think it can swing it that far. This is more like a misfire and unburned fuel getting into the exhaust.
Given that the problem gets better as the RPM's come up, suggest vacuum leak caused by injector seal(s) causing a lean misfire at idle. Could also be plugs, wire, cap, etc.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe