imapd in race condition?

[Marco]

Hello,

   I run Cyrus IMAP v2.4.17-Invoca-RPM-2.4.17-6.el6, with a mailbox  
containing a folder EVASE_C of about 2GB.

I see these two processes that own all cpu resources:

28875 cyrus     20   0  278m  30m 9.8m R 98.1  0.2  73:20.74 imapd:  
ucxx1.xxi.it [10.10.10.53] xx^xx at xx.it xx.it!user.hd_xx.EVASE_C
24535 cyrus     20   0  369m  49m  25m R 99.3  0.4  75:40.36 imapd:  
ucxx1.xxi.it [10.10.10.53] xx^xx at xx.it xx.it!user.hd_xx.EVASE_C

The client is no more connected, but the processes continue to run.
I don't know how the MUA (Thunderbird) starts this sort of race  
condition on the same folder.

Do you have any hint how to deal with this problem?
How could I safely free the cpu load without restart cyrus-imapd at all?

Thank you very much
Best Regards
Marco