I have 80 280zx & had the exact same problem that you are having.
I tried everything that you have & got quit frustrated, until I found the
intermittent wiper amplifier. To expensive at the dealer but found one
at the wrecking yard for $15.00.
You can find it under the dash on the drivers side if it's in the same
place on your car (I don't know) Hope this helps. Good luck.