Speedometer intermittent failure in 1977 911S - it comes and goes

[Orange911S]

I just got back from a 550+ mile weekend in my '77 911S and ran into an issue with the speedometer and odometer.
Yesterday on the drive north from San Simeon to the Bay Area, around Big Sur I noticed the speedo at 0 mph. The odometer had stopped as well. A few times when I got the speed above 50 mph the speedo would register with what I thought was the correct speed for a mile or two then not work again until I got back over 50.

This went on for about 90 minutes / 60 miles then it worked fine for the remaining two hours of the trip. This morning it worked fine again.

I have searched and read a few posts. My question is whether I should go ahead and order the speedo sender now or wait and do a magnet cleaning to see if this solves the problem.

On the other hand, this gremlin may or may not ever return.

Any insights or recommendations?