VpM speed down after 1-20 min

Hi! whats the problem or wrong setup may be in this case:

I start 2-3 projects. Each in own bunch of emails (Catchallboxes, Asiavirtualsolutions and mail.ru with no spam.

In each project I set  90% engines (for tier2).  Machine power is over. No more than 1-3% CPU use (the same digits in RAM).

———

Projects start well with 90-100-300 VPM speed and down and down. 10 min. 70 VPM, 20 min 30 VPM and after coule of hourse no more than 2-5 VPM.

 I tested 50-300 treads in global settings but no result. Speed down fast soon in all settings.

Sitelists and projects – all the same I use before. For example – GSA has keep stable speed 40-60  VPM  from week to week and last days I cant find the problem point.

Any idease please