For a long time now the tachometer on my 300 TDT works intermittently.
When it does work, it works perfectly.  Sometimes it will work for
hours, sometimes it will not work for hours, sometimes it will work for
just a few minutes and then stay dead for a long time and vice versa.

I know about the ribbed small barrel shaped unit on the drivers side
fender under the hood and also the sender unit that is down by the
front crankshaft pulley.

Does anyone have a guess as to which one of these is the likely
culprit?

There is no apparent pattern to the problem.  Bumps don't affect it or
anything else I have been able to notice.  Once when I used a pressure
washer under the car it came on when I started the car afterwards and
it had been dead for a while before that, but I haven't been able to
repeat that, even when I really targeted the sender for cleaning.