The buttons are sticky. On the 300TE, one of the POs children appearantly opened a 2-liter bottle of ginger ale in the car after dropping it -- there is sticky stuff everywhere and I can smell ginger ale when I remove the wood console trim.
One of the things that got sprayed is the pushbutton unit, and you have to hold the buttons down to get it to change modes -- the button for "normal" and "economy" both stick, so when you change to something else, you have to hold the other button down until those pop out.
You can take it apart and spray some contact cleaner (the alcohol based ones ONLY, please, not carb cleaner, it will melt everything!) and see if that cures the problem.
Otherwise, it's a replacement PBCU, new or used.
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!