Debugging intermittent valve failure

My OpenSprinkler Pi has been working nicely for two years with a two-zone system. Last weekend I installed four additional zones for a total of six zones. I tested all valves and they were working fine. However, a day or two later the valves stopped opening when I attempted to activate them via the web UI. When I grabbed a multimeter and measured the voltages at the OpenSprinkler Pi terminals, they were responding as expected. Strangely enough, the valves started working again without me doing anything else.

However, this morning they stopped responding again. What should I do to debug this problem? My guess is that it’s either a power supply, Open Sprinkler Pi or ground wire issue (since all or none of the valves work). Any suggestions for the first few things I should look at?