I have a 1984 240 as well. Mine has about 250,000 miles. Sometimes it dies too. I haven't been able to diagnosis it completely, but here are some things that I have adjusted.
Check your voltage regulator- this bolt on to the alternator, (passenger side of engine compartment). Sometimes I have had trouble with either oil or water leaking down and shorting this out.
Make sure that your Battery is in good working condition, and that the terminals are clean and tightly fastened.
Make sure that your car's idle is adjusted properly. If its too low it could stall out.
Check these things out, and hope you have good luck.
I've been driving a 240 in some shape or another for almost 15 years. They're Grrrrrrreat!
Friday, December 29th, 2006 AT 5:18 PM