Agree with Mysfitz.
You don't want "the viv" to reach the ideal max temp - you need the area where the Corn is i.e. ground level, to reach the ideal max temp. What is the temp on the surface of the substrate, immediately above the heat pad?
Unregulated heat mats can get over 100 degrees within an hour - some can hit 120 degrees. I find it hard to believe that a mat+lamp without a stat, aren't producing temps over 80 degrees.
Also, have you tried another thermometer? It sounds so wrong that yours just might not be working properly.