Replaced the flush valve unit and toilet now flushes fine! I may take the old one apart to see if there is any adjustment that could be made on the time of the second stage of the flush. While I was at the repair I decide to replace the 6" section of PEX (I.D. of less than 0.4") with soft annealed copper pipe (I.D. of roughly 0.5") to see if it made much difference. Perhaps a little more volume...significant? Not sure.