'91 CQ Temp Gauge - when should we be alarmed?

[Christopher Gharibo]

Here is the obligatory summer temp gauge question on an old car. I have a '91
CQ with 210k miles.

The temperature vacillates about the old usual thick line at all times, except
when the ambient temperature is 90 degrees and the AC is on. These
circumstances put the temperature needle past the usual thick line to just
before the 1st thin line. If I shut the AC off at this time, the needle backs
off to the thick line again. The hi/lo radiator fan is cycling correctly. 

Does everything seem to be operating normally? At what point in the gauge
should I be alarmed?

Any help would be appreciated.