newbie w/ ac and speedo ???s

I have a couple of questions about my new(to me) 94 xj.

1. My ac can be running cool, then after about thirty minutes, nothing but warm air for 2-3 minutes, then it gets cold enough to make a meat locker feel warm. I can literally see mist blowing out of the vents. What gives?

2. Traveling up I-75 towards Kentucky last weekend my speedometer read 75mph, but I was passing everyone like they were standing still. I know from past trips in the wife's 04 Altima that you can set your cruise control on 85mph and have to stay in the right lane. I don't think my tire size has anything to do w/ it (235/75/15 Liberator A/T). Is there some way i can calibrate it or have it calibrated?

Did the mist smell sweet? could be the heater core leaking and then stopping all by itself.

Easiest way I know to check the speedo is to run about 20 miles on the highway...check the mile markers at the start and finish points and find the distance traveled compared to what your trip-odometer says you traveled.