Herb, it's probable that I am suggesting something you already checked.
Some software allows you to lock focus between slides. Maybe your focus remained "locked" and the scanner is not autofocusing anymore. Have you tried reverting to default on scanning options?
Something strange happened those last days to my Coolscan 5000. When I wanted to calibrate the scanner with my IT8 target, I aligned precisely the mask over the target and VueScan went on telling me to check mask alignment. It was driving me mad.
When I gave up (after much liver damage

) and made a normal scan I saw that the scanned portion of the slide did not coincide with the mask I applied to the scan. The "cropping" was out-of-sync between the software and the hardware.
I turned off the scanner, re-started Windows, turned on the scanner, "calibrated" it and reset to default the "crop" section of VueScan. The scanner went back to normal operation.
So before sending the scanner for servicing I would simply try to restart the operating system (so as to be sure to clean all libraries and temporary data from memory, also from cache), to turn off and on the scanner (so that internal data regarding focus position is cleared), to calibrate it (so that the internal alignments are repeated from scratch) and to clear any focus setting in software (just in case).
Hope it works
Fabrizio