With the autumn weather settling in the past few days, I've started using the heater. My usual routine is to wait until the temperature gauge has reached its normal operating temperature position, straight up, before turning on the heat. I'd do that, and get normal output, but then after awhile if I want more heat, I don't get it. Even with the heater dial put full on, I get a rather weak heat output. The temperature gauge remains unwavering at the straight up position, and the coolant level is right where it should be. This evening, at the end of a 1.5 hour trip at highway speeds, I checked underneath the hood, the upper radiator hose was hot, the lower cold. Does this mean a stuck thermostat? Remedy is to replace?