I still use fedora 26 for my dispvm. from time to time it breaks. even when using debian. has always been a problem for me since using qubes. the solution for me is just to simply delete the DVM template. Using the qubes manager in 3.2, select view and show inactive hidden vms. then simply delete the fedora dvm. Then recreate it with qvm-create-default-dvm fedora-26.
I also use a cloned fedora-26. I doubt this makes a difference though.
One thing that does bother i've noticed past few month is from time to time, especially if I load it right after machine boots, The dispvm will use 100% cpu and be freezing on me and working slow. And if I kill it, from that point on as soon as I load it it has the yellow triangle. Until I reboot the whole machine. Just seeing that yellow triangle bothers me.
To lessen this i try to give time for qubes to fully boot before I load the dispvm. But it will still happen at times, though much much less frequently, maybe depending on the page I visit.
I don't know what this means though I'd put it out there.
Almost never happened when loading chrome from an appvm terminal in a dispvm. But now its freezing at 100% cpu use everytime I try to attach a usb to it. Whether loading the firefox app from main menu or loading chrome one from an appvm terminal...
wanted to add that adding more cores to the dispvm doesn't help.