One thing you need to do is to detach the interrupt while doing calculations and then attaching them again. See the code here
http://engineerexperiences.com/tachometer-using-arduino.html.
Some more involved code using timers for rpm counting, which is just frequency counting is here
Also, with this sort of code avoid using delay(). Something like this is better
if(millis() > timeold)
{
do stuff…
}
is better.
Also, one thing to be aware of when you are adding servo code is the possibility of interrupt conflicts between the servo and the rpm counter.
Have fun, is your turbine sounding like the wail of a thousand damned souls being dragged down to hell yet? 