Do you see anything across your logging in and around the time you saw the spike? Usually something is going on and I'd bet you have a user doing a big sync job or something along those lines :-)

IN what log would that appear??

It is very hard to trace such activity but if you happen to experience it again, run the network monitor via performance monitor and check which IP pull the highest traffic and match again the IMAP log

Doesnt give away anything....

Crazy. I have only seen it once before on an older buld (85XX)

Unfortunately Brian I can't indicate a single log for this. The source of temporary CPU usage could come from a variety of places, but the most common source is sync, indexing, someone sending to a large mailing list, etc. 

Hello, We are planning to update from 8251 to 8888.

We need to ensure that such issues do not occur after the update.

Could you please share your environment details? 

How many users do you have, and do you use Outlook?

We have about 1800 users, and approximately 1000 of them use Outlook.

Cant guarantee anything. We use outlook on 100% of the clients along with IOS and Android.