Three things to check off the top of my head:

1) /console set maxfpsbk 30
2) /console set maxfps 60 (both of these can be set in graphics options as well)
3) Go change both clients to either DX11 or DX9 and set the graphics sliders to the same settings.

When you set up the group, I presume you used the ISBoxer wizard? If you do think it's an ISBoxer issue, try pasting your config in here and let us have a look.