So out of nowhere, my bed is climbing in temp at a steady rate with NO set temp! Just turn printer on, and away it goes. It reached 135 by the time I noticed the first time.
Disconnected everything, let it cool, turned it back on without touching anything, started to climb again.
Help!!
Sounds like you blew the F.E.T. that controls the bed power. Look to see if there is a RED L.E.D. that comes on when you turn on the printer and does not turn off after a couple seconds. If so you need a New control board.
I just replaced a new Rambo for the same problem. That board was a new replacement from Ultimachine for my original Taz 5 board that had a failed Z axis control.
If this was a new board, let Ultimachine know you had a problem. They have seen a few instances of this on new boards lately and are trying to track down the root cause.
Keep in mind if you order a board from Ultimachine (less expensive than the kit from Lulzbot), a couple sets of pins will not be installed. You can easily add them. Ultimachine might just throw some pins in the box if you ask during the order. They are a great company, in my experience with them.
The Aux Power Out Mosfet is 5v for the E1 fan and Aux Motor Out Power is always on 24V that is unused by default on Taz 5.
Joy 
So, never having replaced the MB before, if I go the route of Ultimachine, after install, do I need to upload a firmware image, etc? And where would I find instructions?
Thanks!
(And thanks for the quick responses from everyone!!)
P.S.
@Nopick, do I simply need to solder a couple of these pin pairs to those points on the board if Ultimachine won’t do it? And are these just jumper points, or are there plugs from the Taz wire harness attached? (I’m sure if I open the case, I can find out ; ) )
Yes, the first thing you want to do is install the firmware using Cura. The Ultimachine firmware that is on there does not work correctly on the Taz.
https://www.lulzbot.com/learn/tutorials/firmware-flashing-through-cura
Make sure you record your E-Steps before removing the old Board.
The Aux Power Out Mosfet connector will have wires attached for the 5V E1 fan. The Aux Motor Out Power is unused on a Taz 5.
The pins are through hole and pretty easy to solder to the board if you have any soldering experience.
I used a sharpie to label similar connectors 1,2,3 and so on from left to right or top to bottom to make installation easier. I labeled the motor connectors x, y, z1, z2, e0, e1 as I removed them from the old board. It is really easy to swap boards… Don’t sweat it. Taking a bunch of pictures before you tear into it can also help.
Thanks folks. As usual, you are great!
Just ordered my replacement board. I’ll let you know how it goes ; )
Fyi , ultimachine has a TAZ version of the Rambo board. When I requested the additional pass through pins, I was contacted and informed. Same price.
I knew they had them, just didn’t know if they could ship them!
SUCCESS! 
After staring at the controller box for a good 30 minutes, examining and working up the courage, I took the leap. I think it took all of 15 minutes to do the brain transplant.
A little freaky when I turned it on, and got no display on the LCD, but system found the board and installed drivers. Once the firmware upgrade was done, the unit rebooted about 10 seconds later.
Updated my E-steps and I’m printing a test print now.
Thanks everyone for your great support (as usual).
P.S.
What do I do with the old board? Toss it? Would IT-Works have any use for it?
It tends to burn the board/pads under the FET making it junk for anything but parts recovery. 
