Loose drop arm

Hi Guys I have a 90 which the steering drop arm continually comes loose causing play in the steering. Ive looked for cracks in the drop arm and found none.

What could cause this? If the splines are worn would a new drop arm help? (does the drop arm wear quicker than the out put shaft on the steering box?).

as it still got the locking tab washer on it?

yep still has locking tab washer

how tight are you doing the nut up?

There was a problem about 20 years ago where there was a faulty batch, or incorrect suppressions, I'm not sure of the details that resulted in people fitting taper spline drop arms to parallel splined steering boxes (or the reverse)