A tiny leak in an aluminum/plastic rad is not to be ignored - especially at the hose connections - as it can become a big, coolant-dumping leak in a second. If it's just a matter of tightening (judiciously) the hose clamp that's another thing. Fix the leak and recheck the overheating situation.

You replaced the 'stat and the behaviour didn't change - that pretty much eliminates the stat. The fan clutch should allow you to turn it by hand but it shouldn't freewheel, ie: keep spinning. If the fan clutch is bad, it could yield these symptoms...when she slows down after working the engine (hills or highway), things will heat up quickly. When she headed out of town last night and saw overheating, was it with a tailwind? Then turned around and met a headwind and cooled down? You can see where I'm going - it's all about airflow through the rad.

Typical 240 fan clutch behaviour is the big roar after startup, then going quiet after a dozen or so seconds, to sound off again when the rad gets hot and the air coming back over the fan is also hot. If it doesn't go through this cycle, it's a bad unit.

If the fan seems good, then we're looking at the classic later 240 temp. gauge problems. Either way you don't want her heading off, worrying about the car.

And be thankful you have a kid who actually takes notice of the gauges!
--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)