There are two big power MOSFET transistors on the board, one for controlling the heated bed and one for controlling the hot end. These transistors are adjacent to the corresponding connectors on the board. The transistors have metal tab heat sinks that stick up above the molded plastic case. If these tabs are touching each other, strange things can happen to temperature control. It's a simple matter to bend one of the transistors a bit to the left and one a bit to the right so the tabs don't touch.
If that's not the problem, there are other things we can check. As a safety matter, there should be no heat coming on until you ask it to come on!
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick