Overheating headache

alg said
I replaced the radiator and put on a factory original fan shroud.

This statement tells the board nothing! What'd you replace the radiator with? A older, more plugged up one or Brand new??

alg said
Monday I took the car for its first short outing in about 18 months.

So almost 2 years now since you replaced it, how nasty was the radiator when installed, 2 years later will only make it worse if it was properly prepped to begin with!

alg said
I am going through the cooling system now in an attempt to determine and correct the possible cause(s).

You should've done that 2 years ago before you put it all back together! But what do I know?