5.0 MPI will not idle

First time user on IBoats. I have a 5.0 MPI which ran perfectly on Saturday - idle and WOT. Yesterday it began to be very hard to start and would not stay at idle at all. When the engine "caught" and I quickly put it in gear at say 1500 RPM, it would run fine - no problem through the power band up to WOT. As soon as I brought it down to no wake idle, it would die, and then I had great difficulty getting it to fire again. This is the same tank of fuel as the day before - full on Saturday - 2/3 on Sunday - same gas station and grade as all this season. I have checked the exhaust tubes on the flame arrestor and changed the fuel filter. The boat is always stored inside a locked garage when not in use. WHAT HAPPENED OVERNIGHT that made it very difficult to start (feels like it's starving for fuel) and impossible to idle, AND, what do I need to do? Thanks in advance from a Wisconsin boater for any input you can give.