Thanks everyone for the replies. I really do appreciate ALL the thoughts and suggestions.

Just a couple of quick clarifications...
--Fuel Temp/cold muffler--We're flying other airplanes at the same time, same fuel storage conditions, etc, and no one has experienced this with other engines (interesting note... we don't have any other evolution 46 winter flyers for direct comparison)
--The blockage is occurring in the pressure line, not the pressure fitting screwed into the muffler
--We tried running it with the pressure line disconnected and it DID run consistently (too lean though). It didn't have enough draw, so the fuel pump idea sounds feasible
--The muffler's pressure tap is already on the upper side

I'm going to talk to the owner about a couple of the suggestions you've posted for me:
1) Disconnecting the pressure line and running a pump
2) Moving the pressure tap on the muffler closer to the engine exhaust (theoretically hotter & less viscous oil in the pressure line)

opjose: It is an engine with very little time on it. That makes me wonder if metal "sludge" could provide additional structure to the viscous (cold) oil, and be the root cause of the problem. I'll have to take a close look at the "exhaust oil" next time to see if there's evidence of metallic coloring.

If anyone has experienced this (especially with a new evolution engine), please let me know.

Thanks again and keep the ideas coming!
--Doug