mike_o, Next time it happens, try pushing the trigger forward. I think the problem is "trigger reset", meaning the trigger must reset fully forward before it can be pulled again. Cause? Could be a reduced power trigger spring, maybe some machining crumbs in the action, a trigger plunger problem, or other issues that impede trigger movement. These types of problems are virtually impossible to diagnose via the Internet.
My own GP100-22 was a real pain …. probably the worst Ruger firearm I ever bought. It had all sorts of machining issues that left it rough. It took a couple hours on the work bench to get everything smoothed out the way it should have been from the factory. Now, my 22 LR GP100 functions perfect.