As everyone has said, have the valves adjusted.
I doubt that your car is going to go from starting to starting only when its warm to not starting in 4 weeks and the car has just died. Mercedes diesels don't die that fast unless something breaks.
Did the car all of a sudden change anything. Noise, smoke, rough running, injector noise. Or did it just stop starting?
Something that no one has mentioned. What is the condition of the fuel? You may have developed algae in your tank and the screen in the tank is blocked. After 20 years old its probably clogged anyway.
What does the inline fuel filter look like? If it is blackish in color, not clear, then you have algae. If it is showing in the inline filter then your tank screen is probably clogged and your starving the engine of fuel.
Do you do any auto work? If so check the inline filter. If not, have the shop check it. IF they find it blackish then have the tank screen cleaned, all filters replaced and an algeacide put into the fuel. You will have to have the screen and filters replaced again in a couple of weeks (2-5 tank fulls) after the algeacide kills the algae the dead algae will clog the filters again so replace the filters.
Our 240D would run rough trying to accelerate at 60MPH. All filters were blackish. The screen in the tank was completely blocked and starving the injection pump.
One other idea is to have the tank removed and boiled out. Usually done at a radiator shop. Your shop can arrange this.
If you go this way then you boil out the fuel tank, replace all filters and then you should be good. Only the initial filter change is necessary.
I put my $$ on the algae problem.
Adjust your valves, the car will run and start better.
Use an algaecide regularly
Change all filters and fluids on a regular basis.
If you have not done the valves then you probably have not followed the normal Mercedes recommended maintenance proceedures. IF you want this car to last forever you need to regularly follow Mercedes recommendations, especially an oil and filter change every 3000-4000 miles.
Here is the product I use. There are others.
http://www.powerservice.com/
I use the Bio-kleen and the Fuel Supplement and Cetane Booster.
Dave
ps Go here to see what diesel with algae looks like.
http://www.peachparts.com/shopforum/showthread.php?threadid=85130&highlight=algae