We bought Sal a mercury vapor bulb (160 watt) and did the recommended 'burn in' period ~ we didn't measure temps during that period since it wasn't near Sal's enclosure. Since we put it in, the temps keep changing. It will be 105F one minute and 99F the next. It's not getting too high (one of us has been home since we put it in and we've been watching the temps every hour) but it won't stabilize. Is this normal for a new MVB?