Sporadically slow MyISAM inserts

I have a simple MyISAM table (8 fields; 22 bytes per row plus c. 40 bytes of index per row). Currently, the table has several million rows - and I run frequent bulk inserts, each of a few few hundred thousand rows. Typically, it inserts c. 5000 rows per second, but occasionally a row takes 5-60 seconds to insert. This happens perhaps every 2,000-20,000 rows or so. I notice that the IO wait in Linux ‘top’ command can go as high as 90% during the insert process, so guess it’s to do with disk writing. Is there any buffer which could help?

It could be flushing key buffer or simply dirty pages from MYD file.

Thanks Peter

Would there be anything which would identify which of these it was, or how would you fix it?

Alternatively - someone at Rackspace suggested that it might be rebuilding the indexes. Would that sound likely?

Many thanks again!