2008 mac pro boots in 10.4 but not 10.10

It certainly sounds like your macOS 10.10 image has been damaged. If you have a Trusted Backup (of YOUR files from that drive, not necessarily MacOS), you can do a Re-Install-in-place. By design, this re-writes over 350,000 files that make up MacOS. It does not bulk-erase any non-System directories, so all your files and Applications are intended to be preserved.

But if your drive is near failure, this could push it over the cliff into complete failure (thus the need for a Backup before you begin.)

I don't know how prepared you are to try different things but I had that Mac and I installed and ran many different systems without issues and thats filling that computer with a sata blu ray optical top bay, two SSD in lower optical bay and two other SSD in the main trays with two mechanical hard drives for data. . So I always had at least four operating system ready to go at any time. I always had one rule I never broke and that was to erase or partition the target drive with the Disk Utility of the system I was going to install onto it.