RAMPs Fail - Hot Mega Polyfuse

I am building my first 3d printer(mini kossel) and I am having a heck of a time with RAMPs 1.4.

Here is the history:

1. Hooked up Ramps included all the drivers and the Smart LCD controller XXL version. Plugged one driver in backwards which ended up frying the driver. Also, at the same time I noticed another driver was not functioning properly. It was doing the jerky back and forth motion and changing the pot didn't help. So I ordered some new drivers.
2. Got the new drivers and hooked them up correctly this time and powered the device on using 12v with the USB also plugged in. Endstops were also plugged in correctly. I was trying to figure out why my endstops were all saying triggered when the SmartLCD began to flicker then die. I immediately turned off the power.
3. Turned 12v back on about 10 seconds later and pop...the linear regulator on the mega exploded and a piece of it when flying in the air. I noticed the poly fuse on the mega was very hot at that point. Mega chip was dead.
4. Acquired a new mega. Plugged the drivers and LCD controller in. Powered it with USB. The LCD lit up then slowly faded. The poly fuse got hot. I immediately unplugged the USB and removed all drivers.
5. I plugged one driver into the X slot one by one with my index finger on the mega poly fuse. 3 drivers functioned properly. One caused the polyfuse to get hot. I discarded the one that was heating the poly fuse.
6. I plugged in the Smart LCD XXL controller by itself and powered using USB. Everything functioned properly.
7. I plugged in the remaining drivers and powered using USB. Everything functioned properly.
8. Hooked up my 12v. Repeated step 4 with 12v power except this time I had my finger on the poly fuse. Everything worked but the Poly fuse got very hot. I immediately unplugged the 12v.
9. I repeated step 5 with 12v. Poly fuse was normal temp.
10. I repeated step 6. LCD came on with Riprap screen but Poly fuse became very hot so I unplugged the power.
11. I plugged in 12v with only mega and RAMPs 1.4 board. Poly fuse normal.

So it seems the SmartLCD is causing the poly fuse to heat up. Is this normal? I am afraid that this may have caused my original problems with the fried mega so I don't want to keep it plugged in too long for fear of fry. What could be causing this? What tests should i perform to test the LCD and/or RAMPs for problems?

Thanks,

Dave

Too much current being pulled through the Mega's regulator is what causes them to blow. Could be a short between 5V and ground, or simply too much current. They aren't designed to run a lot of current, and with all the add-ons people chuck on a RAMPS board, it's put under severe strain unless you power the 5V rail directly and remove the lead in Diode under one of the stepper drivers (eg: 5V from an ATX PSU).