maybe vibration of an oxidized connection? maybe even a poor connection at the headlight buckets, like the socket not fitting the bulb spades well - I don't think they draw a huge amount more, but you can't get more light [and resulting heat] without more amps, and ya never know how much those switch overloads can take, especially replacement ones
I'd try the old bulbs to see if the problem goes away, then check all the connections in the headlight circuit, might be one that can handle the lower candlepower but heats up with higher
Bill
