When I copy and paste a graph from Prism to PowerPoint, the pasted graph is fine at first, but then it changes with repeated viewing.

This suggests that Windows is running low on "resources". Resources are a special, limited part of computer memory shared by all programs. When they get depleted, all programs can act strangely. Loading fonts requires resources, so one symptom of limited resources is strange fonts showing up. Other symptoms are text that goes from vertical to horizontal orientation, labels disappearing (possibly replaced by white boxes), colors changing, and line thicknesses changing.

Resources can get depleted for two reasons:

1. Too many programs open at once. The solution, obviously, is to close applications that you don't need.
2. Badly written programs that take resources, and don't give them back, even after you quit. The solution for that is to reboot. Some people leave their computers running for days or weeks without rebooting; this will gradually deplete resources as programs grab resources and forget to give them back.

Adding more RAM (memory) will not help, since the amount of memory dedicated to "resources" is fixed. Upgrading Windows might help, depending on which Windows you use.