I'd check the fuel filter. Even though you say fuel is running, it may not be enough. I've had exactly this problem today, as it happens, with a 254. (Used professionally) Next time it doesnt start, take out the filter, which thankfully is on a long pipe, and blow through it. This won't make it like new, but it will allow more fuel through than before. If the saw starts now, that was your problem.
254's did have a problem with fuel vapourising when hot, but I thought they'd cracked that.
Norman, November 2008