Single handle Grohe shower valve

Hello, I am trying to figure out what I'm hoping is an adjustment issue--I had my bathroom remodeled about 3 years ago. We installed a single handle Grohe valve (I'm not sure what the model number is), which worked really well.

Lately, water temp feels great at first, but then within a minute or so, it gets cooler. I then adjust to hotter settings, only it feels cooler a minute later again. Within about 10min, I'd be really close to the limiter safety knob.

I have a new GE 75 Gallon heater and I shower about 2 hrs after my kids, so there's really no way it's a water heater issue.

I'd really appreciate your help and suggestions.
Thank you.

scaasi

Could it simply be that as you run the shower longer, which runs both hot and cold to the valve, since it's so awfully cold in Chicago this week, you need more hot water to compensate as the cold gets colder? This is one reason I like thermostatically controlled valves...as long as you have actual hot water, it maintains your desired setting, regardless of the cold's temp or the hot's.

Do you have a hot water recirculation system, because that is a symptom of a "defective" one.