When you tried with E1000 and Intel driver have you used the latest intel driver?
"Windows Server 2003 R2 SP2 ships with drivers that do work, but the ones from intel are newer."*
* https://pve.proxmox.com/wiki/Windows_2003_guest_best_practices
I was referring to investigate inside VM = inside Windows.
If you have also in Task Manager of Windows a 60% CPU usage is it here the load need to the investigate from the tab Details, order by CPU%.