I had almost the same experience in the fall on a solo century. Kept going slower and slower on a charity ride that I hadn't done in several years. Thought maybe I was not remembering hhow tough the route was, or perhaps that they had picked a different more challenging route. I was so aware of the time cut off at the 30 mile point and was in tears by the time I pulled into the first rest stop. I checked the tire, couldn't find anything and so pumped up and started off again and was struggling hard again after about 8 miles. Finally stopped and changed the tire, gave myself a pinch flat, started to change it again while people rode by me with no one offering to stop or help. It was really discouraging and I was about to call the ride and give up when a ride marshall stopped, whipped the tire off, changed the tube , popped the tire back on and gave me a pat on the back as he rode off. I still didn't make the cutoff time, but after the tire change I did manage a decently paced and mucc easier and less stressful 80 miler.

Sometimes it is just a matter of timing and people and the phase of the mooon as I have since done several other centuries on chariity rides, again on my own and have no problems with tires or wind or mental exhaustion at decent pace.

You and your bike each diserve a gold star for your recent century.

marni