First thing you should look at how your browsers are closing. I suspect that your browser instances on the CI server is not getting closed.
Also, implement the Grid if you have not done yet. This will help you distribute the load and more stable tests.
And also you should look at your perfmonitor and see if there are any existing chromedriver.exe process running. Too many of those process will slowdown execution and you may encounter the error above.
Clean up your %temp% folder and retsart your CI server once.
Hope that helps!