OE outbox not clearing sent messages

Hi,
I’ve got a problem with Outlook Express.
Win 98SE, initially OE6, removed and reloaded OE5.
Problem that recipients complained that messages were being sent many times. Found that message stays in outbox and when SEND/RECEIVE happens sends them again.
Inadvertently upgraded OE6 to SP2, no access to the internet, hence revert to previous installation (OE5). Still the same problem.
Found that INBOX, SENT and DELETED boxes were huge ~ 200Mb. Renamed all to xxold and created new ones. Problem went away and I assumed this was answer.
Now has started again after a short time.
MS knowledge base says the lack of HDD space will cause this, but there is Gigabytes of free space, so unlikely culprit.
Any ideas before I go on site?