max temp/ firmware issues on workbench (Page 2 of 2)

If you have a barrel connector with wires on it you can test with any 12volt power supply.  (Think car battery)
If your willing to cut your barrel connector from your PSU you could test with that connector.
Or you could even open up your PSU and inspect for bad solder connections internally.  The problem with restricted current is that the voltage is still going to be 12v, there simply isn't enough of it to feed the demand.  The stock PSU has 3 12v outputs (2 for bed and 1 for board). Now I haven't verified this, but am willing to bet they are all pulling from the same source internally.  On all of my printers I choose to run a dedicated PSU for my bed heaters.  I still use the board to control it via SSR (relay).  This frees up the board PSU to handle only the motors and hotend.
The problem with your scenario is we've pretty given all the common suggestions.  Now it's just going to take good old trouble shooting and the process of elimination to find the source of your problem.  So before doing anything, re-read this topic and be certain that none of those possibilities are the cause.
There is nothing more frustrating than tracking down a power issue.  Rest assured, we will figure it out but it will take time.