[Exim] Restricting queue runners handling retry

I've had some odd occurences on our mail hubs ... when it gets to the retry
time of several messages at once, a lot of messages coming into the queue
pile up behind them, increasing the length of the queue by 10 to 20 times
it's original size (usually below 20Meg with 25-35 messages on both hubs).
Is it possible to restrict the number queue runners handling messages that
have reached their retry time, or keep a certain number of queue runners on
first_delivery?
If this isn't possible, could it be a late submission to the exim 4 wish
list?

Zed