Early 2008 Mac Pro Boot Loop KERNEL PANIC

Your startup disk has become set to a Yosemite Install image, and that image may be damaged.

Try holding the Option key at startup, to get the Startup Manager, which over the span of up to five minutes will brute-force discover every potentially-bootable Volume and draw an icon for it. select the one you really want and tell it to proceed.

If that gets you out of the loop, As soon as MacOS is running, set the startup disk to the one you want daily.

If that does not get you out of the loop, you may need to choose a Recovery HD and repair your regular drive. It is possible you may need a re-Install.

The Apple logo is NOT even in the computer at Startup time. It is loaded AFTER committing to boot from a particular drive, with the first "blob' of software from a magic place on the Boot drive.

The startup Manager is in the computer's Read-Only Memory (ROM). Holding the Option key means that NOTHING is loaded from any drives. They are casually examined for bootability, only. Then it draws an icon for each, and waits for you to provide direction.

If your keyboard is connected to a USB port directly on the chassis, and not on a display or other hub, Option key startup is one of the simplest and most predictable things your Mac can do.

If the keyboard is connected through a display or any other USB Hub instead, its input will be seen too late to change anything -- and, Kaboom.

There are several important things that safe mode does differently:

it boots up the kernel, then does one pass of Disk Repair as it starts up, then loads MacOS. That can take an extra five minutes.

It loads a Minimal set of Apple-only extensions, excluding graphics acceleration.

it takes defaults for as many things as it possibly can, but changes you make will "Stick" back in regular mode.

If you get a good display (screen re-draw will be wonky and slow, but ultimately correct) basic graphic is working. But if it does not work in regular mode, you may have clobbered part of MacOS bring over your files, or you may have some tension that is fighting with MacOS. Or your graphics card may have issues.

If you imported an different version of MacOS, it may not be compatible, or it may require firmware changes to boot properly.

To use those 'snag keys' (such as the Option key) your keyboard must be connected to a USB port on the chassis, not on a display or any Hub.

The snag keys are evaluated first, before anything is fetched from a disk. Your kernel panic occurs when something is fetched from a disk.

<<The screen I got after the DVD booted was a scramble of white space and weird character plus empty space at the bottom half of the screen.>>

That certainly sound like the accelerated graphics on that card is failing.

The Mac Pro with Xeon processor does not need to have its memory tested extensively. It has error-correcting RAM memory.

RAM Memory problems do not fester. it continues to work and corrects errors on the fly, until it get a double error, then it halts with a distinctive kernel panic.

User Tip: Mac Pro and Error Correcting Co… - Apple Community

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<<The machine just froze after 23 minutes of testing>>

Mine appears completely dead a few minutes after the test starts. I believe this is because it is hammering on the memory so hard it does not allow even a single cycle to be used for screen updates. After about two hours, it finally awakens and says 'no fault found'.

The ones at the back are directly are on the mainboard.

The ones one the front are on a peripheral board often referred to as the front-panel board. That is one step more complex, but is supposed to work as well.

As soon as you power up the Mac press and hold the option/alt key down and that should force the Mac to boot to the Startup Manager, as Grant has already mentioned.

As you have a MacPro do you have more than one operating system installed on a different disk from the one it is trying to boot from. For example if you know where that Install Yosemite.app is located and on which disk, and only if you have another disk(s) with other another OS installed. Then take out the disk that has the Yosemite.app on it, power up the Mac and it should boot up to the other disk that has an OS on it.

For example, I have a Mac Pro with 4HDs installed and each disk has an operating system installed.

If I have the disk in Bay 1 with El Capitan on it as my start up disk, and if I were to experience a problem similar to yourswhere I get a kernel panic, I could take that disk out. Then when powering back up without that disk in place. the Mac should boot to one of the other disks that has an OS on it.