For example, if you type "dif" in our lookup it only shows up one record.  (just typing the letters at a pretty normal rate)

However if you type 'd' then 'i' then 'f' waiting for the ajax call to finish each time you get (correctly!) all the results that should be found

This seems to be a pretty significant bug in how your ajax calls (or whatever you're using) are being handled and suggest your debounce time on this control needs to be increased or (if it doesn't exist) added.

In the first case, it appears to only do one ajax call (ie, we only get the spinning logo once) so my guess is that you're doing that ajax call only on the first letter we've typed ('d') but then the client is filtering to all the letters we've typed.  This would actually suggest you have no debounce time on the control at all.

Thanks

Marcus