xshift28x

I have a quick question.....I have a 96 civic dx and when I'm up around 80 mph and press on the brake the whole steering wheel starts to shake. Any idea what this could be. Also, sporadically when I run the heater or air (not a/c just vent) my car will push to a really high temp but the minute I turn it off it goes back to normal. Any ideas on this? Like I said it only does it sometimes not everytime I have the air or heat going