temp guage/heater question

'02 XLT, 3.0 L. The other morning, outside temp was in the upper 30's. Driving about 10-15 minutes and noticed I was still cold. Temp knob was on HOT (max), cool/cold air coming from floor vent. Noticed engine temp gauge was still on C. Just then, I hit a bump, engine temp gauge jumped to mid-range and HOT air started coming from the floor vents. What's going on?

'02 XLT, 3.0 L. The other morning, outside temp was in the upper 30's. Driving about 10-15 minutes and noticed I was still cold. Temp knob was on HOT (max), cool/cold air coming from floor vent. Noticed engine temp gauge was still on C. Just then, I hit a bump, engine temp gauge jumped to mid-range and HOT air started coming from the floor vents. What's going on?

Sounds like a stuck open thermostat. When was the last time it was changed???

Doh, I totally overlooked that possibility. I thought of a stuck closed t-stat but that would make it overheat. It's been behaving the past week but I'll change it pronto. Thanks!!!