Make sure the system is in closed loop mode (see Steve Brotherton's DIY article on electronic engine controls) when setting the mixture. If the heater on the O2 sensor is bad, it will be in open loop mode when you set it, but will go into closed loop when the exhaust heats up while driving, and I think this is what happened to you -- it was set way too lean in closed loop.
A defective o2 sensor or a bad connection can do this too.
The O2 sensor is used all the time to control the mixture except when it is too cold on initial start up -- that is, it is only NOT used for that first couple minutes until the heater gets it hot enough.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!