Mac Pro freezes on start up, normal proceedures tried

Same here

You are describing exactly what i go through every single day. I have had this same problem since 2010... haven't find an answer yet.. apple ppl have no idea of what is going on... from time to time when it does run, i go for an update, but there's nothing to update... so, it has been almost 3 years, where my thought has been "8000 bucks thrown away to apple and they haven't done anything to fix it"

Maybe memory

What you're describing sounds to me like a memory problem.

You mentioned opening up the case and swapping stuff around, so you sound reasonably confident with the insides.

Try this, but I don't guaranty the procedure, and the usual disclaimer about doing stuff at your own peril....

There are two slot-in daughter cards near the bottom towards the back, firmly pull out one card at a time. On one or both of these cards will be your memory sticks, make a note of their positions.

(be careful about static electricity. Make sure you touch the metal case at intervals to discharge any static that might be building up on your body. The materials in your clothes and carpets have a big impact. Occasionally touching the case or something else metal that you know is earthed will keep you free of static and protect the electronic components you're going to be handling)

Now I'm going to suggest you pop the memory cards out and put them back in reverse order (change the slot the memory cards are sitting in). Pop them out by pressing the tabs/levers at the ends of the memory stick sockets.

Whilst the memory sticks are out, you might want to consider cleaning the dust too. Another tip is to take a pencil eraser and lightly rub it over the gold contacts along the bottom edge of the memory stick. Don't go mad, be gentle. Blow off any residue. This helps remove grime, grease, etc, and helps with the electrical contact when you re-insert them.

Now put the memory cards back into the slots, just change the order. If they came out as A B, put them back as B A. Press down firmly at both ends. You can check the position of the levers at the end. If you've pressed the memory stick in correctly the levers will have snapped back to their upright position, fitting snuggly against the side of the stick.

Now consider swapping the daughter cards as well. Top becomes bottom and vice versa. You don't have to do this if you only have a small amount of memory (e.g. If there's no memory on the bottom card, there's no point)

Make sure you push the daughter cards home firmly.

Try to start up your machine.

If this doesn't help, sorry. If you're obsessive you might consider reversing the change and putting everything back to the way it was. (You shouldn't need to).

Since you talked about dust, you might also find the heat sink fins on top of the CPUs are blocked. This will stop the cooling air flow, and may lead to overheating. If you're lucky, your mac has been detecting the overheat condition and shutting down. If you're unlucky, you might have "cooked" the CPUs.

Don't panic yet.

You can take the front fan assembly out, and then use a vacuum to clear out the dust from the heat sinks. Search the web for mac pro CPU upgrades. There's some good self help upgrade instructions with pictures out there. You just need the bit that shows how to remove the front fan.

Tip. Looking from the side, the front fan assembly is the first grey plastic block in the bottom left corner. It's held in position by a Philips head screw on its top right corner. It's usually very stiff when you first try to pull it out, it sits on a chamfered slot and rail at the bottom, plastic to plastic. Just pull hard after you've removed the screw.

Once it's out look to the right in the gap, you'll see the copper heat sink fins, probably with a lot of dust. Time to get cleaning.

Now reassemble, just locate the fan assembly rail into its slot and slide home with a firm push. Put the screw back.

Now try and restart.

One other thing to try, is resetting the PMU. Search Apple's web site for the instructions.

I hope this helps. Otherwise you'll need to get it checked,over by the experts...