Over heating heaters

[washefuzzy]

I have two 50 watts heaters. I wanted to see how stable they where so I tried them out in a 5 gal and 2 gal tanks. The heaters would work fine for hours and than suddenly would spike right up into the 90"s. Arggg!
The question is: Are the tanks to small for a 50 watts heater and is this what is  making the heaters act up? I was also wondering if the heater touching the bottom makes a difference?