Prints Head stops lifting on prints exceeding 30 mins

I rescued (3) maker-farm Pegasus 8 units that were originally used in in school stem class for about 2 years from the dumpster (1) was assembled by the instructor, (1) was assembled by the students (this took them about 4 months), and the 3rd never made it out of the storage bin and was raided for parts when the first two developed issues I have been trying to refurbish them and use them for some small projects, however the problem I seem to have is the unit I'm currently working on is litterly only good for small projects.

It can pint out test cubes fine and any small job like that, I recently tried twice to send a print that was 140mm Long x 40mm Wide x 8mm High and both times when the print head got to about 3-3.5 mm the z carriage stopped lifting and drove the print head through the job.

Right now I am printing 3d letter stamps for my son's latest hobby, 15mm Long x 15mm Wide x 8mm High and if I print them one at a time they print flawlessly, but I tried to speed up the process by printing 4 together (group to the center of the bed) and just like clock work at about 30 mins into the jobs about 4 mm high the z carriage stopped lifting and drove the print head through the job. So I have resumed single little prints and have no problems.

I'm speculating that based on the fact that it seems to happen at about 30 + minutes into the projects that perhaps something is over heating, but that guess it based on my knowledge of computer repair, typically systems will shut off or reboot if they have overheating issues. So maybe when the printers been printing for over 30 mins the Ramps board perhaps over heats, maybe the z stepper driver chip?

however I could guess all day long, but would like to hear from others, I doubt I'm the only person to experience this phenomenon.

Thanks