Try unindexing and re-indexing (gw -unindex, gw -index), or using a later release if you have it (19990701 or later). An alternative indexing scheme that should work around it is to set indexblock to a value larger than the largest file being indexed. This would require writing a small Vortex script:
<DB=/path/to/your/db>
<SQL "drop index xhtmlbod"></SQL>
<SQL "set indexblock=10000000"></SQL>
<SQL "create metamorph invert index xhtmlbod on html(Title\Meta\Body)"></SQL>
Remove the "\Meta" part if your database does not include META tags.
