This might have something to do with the way the control unit is interacting with the throttle valve switch and/or the deceleration shut-off microswitch. The throttle switch has idle, run and full throttle positions. It shares a common ground with the control unit and the deceleration microswitch which is also an input to the control unit. Plenty of room for mischief.
So maybe you are driving along in closed loop there is a malfunction in the EHA. You floor the throttle and the throttle switch kicks the system out of closed loop and somehow the EHA gets its act together. Or maybe the decel switch is sending a signal telling the control unit to shut off fuel. Then full throttle grounds tells the control unit to stop this nonsense. And maybe all of this is caused by moisture?
I am struck by the coincidence of the "full throttle cure" and the full throttle switch in the injection system.
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