This problem has been around for a long time and it is very easy to fix without any tools that i know are available, also foxfix is a great tool but does not always fix this particilar problem.

Firstly in my experience, the problem stems usually from large memo files that are updated reguarly so try packing memo files often. What has hapened is the record pointers for the table, memo file and index file have gotten out of Sync so when you issue a PACK command FP/VFP cannot complete the task because it cannot find a record on the memo file and then issues the incorrect error, the memo file is not invalid the index file is.

Know to fix the problem, open the table (you should be able to do this as you described it is only when you pack the table.).
Issue a COPY STRUCTURE TO command (ie make a copy of the table) open the table copy and then APPEND from the original. Of course make sure set dele is on. You will find this will work in most cases. You might want to make a log of notes added to a memo field. What I mean by this a seperate table that has a field for a key, table reference and the memo note. Save only the individual note added. the user interface has to be set up that you can identify the individual notes added but thats not a big deal. Once you have done this then a rpogram can be written to rebuild the notes from the log file.

I hope this helps .