Why when the temperature is above 65 degrees outside my engine surges

When stopped at 1000 rpm and in park at 1200 to 1500 rpm?

It runs a little above normal but not too hot during this time also. I do hear the fan kick in when the temperature starts going above normal.

At night or evening it runs fine. The engine temperature is steady and the rpm stays around 665 rpm.

The vacuum and compression are OK. It has new plugs and gas filter (4 months old), oil changed (1 month ago). No bubbles in coolant, oil looks normal.

I have been experiencing some coolant loss adding about a quart every 2 to 3 weeks but haven't noticed any leaks.

The upper and lower radiator hoses are hot. I replaced the radiator cap but still experienced the problem. I have a small crack in the filler cap (has no plunger) but I've never noticed coolant leak from it. I have to special order a filler cap.

NOTE: The problem was nonexistent during the fall and winter months.