2000 Jimmy -blower control problems?

When placed in defrost and on high, the fan will slow down and speed up on it's own(not set in auto). Occasionally, it starts to blow air from the dash vents while still in defrost mode? Is this a vacuum problem? If so, is there a common vacuum line I should check?

Thanks, Rick

When placed in defrost and on high, the fan will slow down and speed up on it's own(not set in auto). Occasionally, it starts to blow air from the dash vents while still in defrost mode? Is this a vacuum problem? If so, is there a common vacuum line I should check?

Thanks, Rick

Could be vacuum problem.
Check vacuum line from engine all of the way to heater controls for full engine vacuum and or leaks.
Watch for cracked line and lose fittings and leaking vacuun ball if it has one of them.