One thing to check is the vacuum line from the switch to the bypass valve. I had that occur on my SL and that is what it turned out to be. Check out our website at
www.mbca.org/northtexas, go to tech stuff, then engine technical, there is location of engine controls. Fully inspect the vacuum lines to the bypass valve, the switch, and even check the valve to see if it is operating. Get this fixed as if it is broken you could ruin your air pump, or even worse ruin the converters. Good luck!
Brian