Strategy tester agents running progressively slower

[thmsk]

I have a side machine with 8 cpu cores that I use to run strategy tester agents. It has metatrader5 on windows server 2012 with nothing else running. I've set up 8 agents with the strategy tester manager. When I run the first couple optimization tests everything goes well, cpu maxes at 100%. But then as I run more tests the agents slow down, cpu maxes at 20%. When I delete and add the agents again, it goes back to normal, cpu at 100%. What could be causing this? I have other PCs running strategy tester agents but they don't have the same problem.