We have a Broan Allure III hood range that was installed in 2003. It has a three-level light system and a five-level blower speed. Both are controlled by front mounted push-buttons. It is wired into a 20amp wall circuit that also includes one kitchen receptacle and three garage receptacles. It is an un-grounded circuit. The house was built in 1957. 
Since the hood was installed, it has exhibited the following odd behavior: With no load elsewhere on the circuit, the light has only two levels, low and high. No mid-range. Further, when starting the blowers, at any speed, the lights go off and the blowers do not work. Nor do the blowers start even without the lights on. However, (and this is the good part!) if there is a high load elsewhere on the circuit, both lights and blower work perfectly. We've gotten around this by turning on the toaster that's plugged into the kitchen receptacle. Once the blower is on, we turn off the toaster and both blower and lights work fine - Three light levels, all blower speeds. Only a high load seems to affect this. A light won't work but the toaster or an espresso maker will. Both pull over 1000 watts.
We thought the circuit board in the hood was bad so just recently had it replaced. Same problem. Electricians are stumped. Any ideas as to where to troubleshoot next are appreciated. Barring a solution I guess I can get another brand, but my wife likes this one and it works in our kitchen design.
Thanks.