I think you answered your own question. The fan not turning is suspicious. Do you have a voltmeter? Can you check the actual voltage being supplied to the board? I'm not sure how big the heated bed on the Play is, or how much power it requires, but your symptoms have all the hallmarks of an inadequate power supply. Some cheaper power supplies don't really achieve their rated power.
There's a chance that your power supply might be better behaved if it had some load on the 5V rails (not all supplies behave that way, but some do); maybe you could wire up some 12V or 6V incandescent light bulbs to it as "dummy loads" on the 5V supply and see if that stabilizes your 12V supply. (Running the bulbs on less than rated voltage simply means they will be dim and last forever.)
But the surest way to have plenty of power is to use a supply that's rated for at least 29A on the 12V rails. Often the ATX supplies actually have two 12V rails, and the way the Rev F board is wired, they are paralleled - so if they are rated, say, 19A each then you actually have 38A total available to the Printrboard.
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