The Vehicle
2003 Protege5 w/ 120k miles, recently changed timing belt, water pump and thermostat.

What Happened
Driving in high 40 degree temperature tonight when out of nowhere I noticed the temperature needle rising slowly while doing some basic, casual driving- nothing high revving.

The temp needle went up to about 85-90%, then came down a bit. I made it home two minutes later and spent the following two hours trying to get it to overheat. I was only able to get it to overheat once more, but it was completely random.

Details-
- Fan was running almost constantly
- No bubbles in radiator & filled to top.
- No leaks
- Small puddle of liquid formed under exhaust pipe after getting it to overheat the one time

Any idea what might be the issue?