2000 GMC Sierra - Erratic loss of electrical power

During the past week, I've noticed electrical power being lost completely and regained quickly at different times while driving. This mostly happens right after start up, or on bumpy roads. What happens is the ignition panel warning lights come on, radio cuts out, engine begins to die, ignition chime sounds, etc, as if battery connnection was lost and regained very quickly. I've checked that (with truck running) and connections are fine, along with other electrical box connections under the hood. Truck runs great other than this. What could be the problem? Computer? Ignition switch? How to troubleshoot?

Many thanks!