240D Cold Start?

Question for the board:

I have a 1976 240D with 340k miles. It seems that every time the ambient outdoor temperature slips below 75F, the car will not start or takes a very long time to start. The situation becomes worse if the car sits without running for more than a few days while the temp is below 75. I replaced the glow plugs today to no avail. I've tried using the block heater, but this appears to have no affect. The only correalation I have found to this car not starting is the temperature. It starts fine in any condition during the summer months. Thoughts?

Todd