Why does car engine surges in weather above 70 degrees F.

My engine surges when the outside temperatures are above 70 degrees, 100- rpms when stopped, 1200 to 1500 rpm when in park.

Someone from 2carpros suggested checking my ECT.

When the outside temperature is above 70 degrees F my engine surges at 1000 rpm when stopped and 1200 to 1500 rpm when in park. I have no problems at night, during the fall and winter or when temperatures are below 70.

An Expert on this site suggested checking my ECT.

OK. I checked the ECT with a ohm meter. I used a chart online as a reference. This is what I read:

68 degrees F - I read 2.3 k ohms, the chart read 37.30, my ford manual states 2200 (2.2) to 2700 (2.7). Is the Ford manual accurate?

104 degrees - I read 1.14 k ohms, the chart read 16.15

122 degrees F - I read.60 K ohms, the chart read 10.97.

140 degrees F - I read.50 k ohms, the chart read 7.70

I've not tested it at engine temperature (180 degrees F) but the ford manual states 290 (.29) to 350 (.35). I don't know if this is accurate.

Please advise.