isamchk -fewo giving strange messages. Should I worry?

I run isamchk -fewo <TABLENAME> (on MySQL 3.22.21) and get the following disturbing messages:

Found block with too small length at 94147; Skipped
Found block that points outside data file at 94152

These messages appear hunderds of times for a table with about 750 rows.

It probably has to do with the primary key index since they keep on coming when I remove all secondary keys.
They also seem to depend om the amount of defined fields in a table and appear more often when TEXT/BLOB fields are used.

Should I worry? Are these messages mere messages or real errors? And how can I prevent getting them? I went though previous posts in this group and studied the online MySQL manual but found nothing.

Actually I worry already since the customer I have programmed this application for is getting index problems
once every 3 months or so. And is losing data. A big headache-inducer.

Thanks in advance for replying.