Paul:

You didn't say what the OAT was. I'd gather that it was above 0C.

I fly a PA28-180C (PP-SEL/Instrument). This past summer in severe
clear, 2 SMOH, I had a rough running engine and made a 180 and landed
right back at the airport I departed from (from about 10 min out). We
looked at everything and the only thing we could come up with was I had
gotten a load of ice (for those of you who have not flown this model,
the updraft carb attaches to the intake manifold which runs through the
middle of the oil pan -- difficult to get ice, but I managed).

In your case I am thinking that you need to pull and inspect your fuel
filters and/or screens. You may find them contaminated with water (or
perhaps some other foreign substance).

But then, just like in my case, static run ups, sumping fuel, etc.
shows nothing. And the plane runs just fine like it was all a figment
of your imagination.

As another person noted (being much more familiar with your model than
I am - I can only drool at the M20 parked over at the maint hanger),
you may have pockets in your tanks that allow for trapped water and by
luck of the draw...

BTW - This PA28-180C is the first I've owned. Used to fly rental
Cessnas which seemed to always have water. This one has to fly through
a serious rain storm to get any water. Seems it sucks it in through the
tank vents!

Later,
Steve.T