I've done a little bit of research I've into plaster (calcium sulfate) and it seems to me that it'd have a much greater impact on your GH than on KH or pH. On the one hand it seems like a likely suspect, but I'd keep looking just in case… new decorations? new rocks? could it be a seasonal fluctuation in your tap? How much of a swing are you talking about? How many plecos?
But agree completely with anona… no need at all for a weekend feeder, just polluting the tank. I've left my guys alone for up to a week with absolutely no harm done, and wouldn't blink at leaving them for a long weekend.
Fish have an entirely different metabolism than we do. They don't need that much or that often.
Many have adapted to the feast or famine conditions that seasonal variations create in nature… they'll always act hungry, even if they're well fed. That doesn't mean they need to be eating constantly, it means they'll gorge themselves while the gettings good because it might not be tomorrow (or the day after that or the day after that).
Skip the feeders, but keep an eye on other potential sources for the pH bit.
I lost three small pleco's on that one. 
Also, the 'pH Down' solution that came with my test kit didn't work - I bought a different brand that worked.