Had a similar problem with an American V8 once. Except instead of a hairline crack, it had Nitrophyl (plastic foam) floats (as I think the TR250 carbs do). The old floats were not resistant enough to ethanol (or might have been MTBE then) and had absorbed fuel until they were just a bit too heavy. Most of the time, they would still work enough to keep fuel inside. But when things got hot under the hood (like pulling a grade), fuel would start pouring out the top of the carb.
Hot fuel is less dense, so the float has to displace more of it to hold the valve shut. My theory is that is why it worked sometimes and not others.
I replaced them once, the new floats worked for a few weeks then the problem came back. Apparently, at that time, even new floats weren't ethanol resistant. After much searching, I finally found some brass floats that solved the problem permanently.
I'm assuming that, by now, no one is selling non-resistant floats in the US. That's why I mentioned replacing the float first thing.