[Exim] Exim go slow

Hi all,

I am experiencing an occassional problem with Exim. I run a few reasonably
large lists using Exim. The processing speed of Exim is very well suited
to list serving, except on random occassions when Exim, contrary to
configuration, processes a given list sequentially. This can blow out the
delivery time for a given list to 4 or 5 hours. My configuration reflects
my requirement:

queue_smtp_domains = *
queue_run_max = 15
remote_max_parallel = 100

I am not hitting the hard process or open file limit of the operating
system. The msglog shows that a message from an 'affected' list to a
particular MX around once a minute. Yet most of the time I am processing
up to 200 or 300 a minute.

Has anyone experienced this problem and found a solution? I could not find
mention of it in the mailing list archives.

Thanks for your help

Gavin