I was returning to Frederick, Maryland, from Winter Haven, Florida. My Velocity’s Lycoming IO-360 had been unusually hard to start in Winter Haven, almost causing me to miss my IFR departure window. After a quick break and fuel stop in Waycross, Georgia, the Velocity was again very hard to start. Approximately halfway between Waycross and Lumberton, North Carolina, the engine shuddered, and then returned to normal. Scanning the instruments, I found nothing unusual. The engine then began a vibration that was anything but normal...