Rough starting hot and cold

My 88 300 SEL is starting hard and running rough for 30 seconds or so. It runs good after that although get fuel smells. I have adjusted the idle mixture (I know, but the previous owner damage that I am trying to fix) to about 45% duty cycle. It will read ~7v (pins 2 to 3 on test jack) after starting, then gradually drops to 2 volts. Without the engine running and plate in idle position I get 70%. Partial deflection and I get 90%. Full deflection and it stays at 90% duty cycle. The lambda voltage is down to .6v (sensor wire to ground). If I try to adjust lower, it drops to zero. I unplug the throttle valve switch sensor and read the three pins:
1-2 0 ohms (idle)
1-2 0 ohms (full load)
2-3 open ohms (idle)
2-3 open ohms (full load)
2-3 open ohms (partial)

If I adjust the mixture it will start perfect for a couple of times and then gradually return to bad start. I think I have a bad throttle switch. Can these switches be cleaned or renewed vice replace. Where can I get just the throttle switch cable? What else might be wrong with these symtoms?

[Benzmac]

Be sure you don't have leak down in the damper under the air cleaner or in the accumulator that is located with the pump package.

Thanks for the response. As far as the throttle position sensor reading bad in fuul load position even without the engine running, is that mean that it has to be changed or will a leak down in the damper under the air cleaner or in the accumulator cause the same problem?