overheating, not overheating

Hi all. Can't find an answer to our problem so here it is. 98, Temp gauge never gets passed 3rd line, driving normal speed, no problem, driving slow, it sounded as if it was boiling (luckily we were in the driveway), added coolant to resevor and took it to radiator shop, guy said it was dirty and flushed it, driving normal (for days), no problem, driving slowly (looking at blackberry patches), overheated and left us stuck in the middle of nowhere. Mind you, the gauge was still in it's normal place, the 3rd line. So I'm thinking thermostate. Good guess? I've changed them on older vehicles, any harder on a Caravan? Anything I should look out for? Thanks in advance!