I second Art. Your 240, like all '85 and newer, has a two piece crank pulley. The two pieces are isolated from each other with a rubber bushing that eventually dries out and cracks, allowing the outer part of the pulley to spin independently from the inner part.

Do this: Make a line across the two parts with some paint or something. Start the car and go for a drive. Punch the gas a few times, make some sharp turns and turn on some electrical accessories to put some extra stress on the crank pulley. Now, cut the engine off and look at your marks. If they no longer line up, then you are in the market for a new pulley.

I just got done replacing mine. It would make noise and my dash lights flicker when cold or upon hard acceleration, but would catch when warmed up a bit. Mine was so worn that I could see it wobble a bit when I punched the throttle from under the hood.

There was a thread on here recently of a guy who replaced pretty much the whole charging system on his 240 only to find that his problem was a bad crank pulley. Learn from the learned.

Good luck,
Charlie

P.S. If this is your problem, then be sure to look up Bruce Young's rope trick for removing and replacing it. Works great.
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1985 244DL B230 M46 170k