These sorts of things usually end up being a hardware issue. Whenever I see truly random computer behavior, I suspect the power supply first. Do you have any spare parts to start swapping? If not, this could be really frustrating to troubleshoot.
I solved this by checking the "auto-flag confirmed alerts" checkbox in the Sentry config and using the "view flagged only" setting in UI3. It's suboptimal, but works okay unless you were using alert flags for something else.