Why hang PPPoE clients?

Hello!
Tell me please, can someone faced with a similar problem?
We have to authenticate subscribers four NAS.
Three of them - PC x86, one - RB1100A
Mikrotik version 5.19 and 5.22
Using the CPU up to 50%
Why periodically hang the session, caller hung up, but ppp connection remains.
Thus there are 2-3 sessions on different NAS.
In what could be the problem?

Thanks.

check dbug log -ppp (pppoe) . Does it tell annyting?