It sounds like you've hit a bug, and whether you have or not, your dilemma is a frustrating one. We feel your pain. Well, please don't let this insult your intelligence, but sometimes it doesn't help to restate the obvious...
scandisk
repower the machine + 10 seconds (not just reboot)
isolate the offending record(s), if any, by breaking up into sections as you described
run Group By queries to see if weirdness appears in the data
try copying the table to another table (or make-tabling to another table) and try again.
The last suggestion shows my resignation to the power of voodoo, in some situations where only a supernatural solution appears to work! :-Q
Try to let us know if you figure it out. Again, "we feel your pain." -------
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