I had a similar problem with my lads 2003 bike...I had a spark, fuel, ignition timing was ok but would only run for a few seconds....turned out to be a distorted reed block which meant the reeds were only just sealing but not completely. I contacted GasGas through a spanish speaking mate and they admitted that they had a batch of 'out of tolerance' reed blocks. A new block cured the problem completlely and the engine ran sweeter than any TXT I've heard since.
I would definitely check this before I started changing ignition systems etc.
Andy