recent change in temperature

[hon.RML]

Abrubtly, while sitting in traffic the othe day the temp rose to about 230 after idling for about 10 minutes. It dropped to the 200 range after about 10 minutes of freeway driving. The ambient temp was about 55. Since then the temp has been rising to about 210-220 even during freeway driving and up to 230-240 in traffic. I used to encounter temps in the 220 range in traffic during hot summer days( in the 80's and 90"s) but it would generally run in the 190-200 range at freeway speed. The car runs fine, haven't noticed any appreciable change in performance but the engine is definitely hotter.( I replaced the gauge 2-3 years ago so presumably that's not the problem.) Should I be looking at the thermostat ?Is there any other explanation for the temp spike? I've considered replacing the trombone cooler for a while because of the elevated temps in the summer but presumably that's not going to resolve the problem when the summer heat waves return.