Hi Paul,
I had the same problem and it took some time to identify the cause. I changed the battery which made no difference. I also checked the voltage at the motor terminals when starting and this was OK as well. However, I was also lucky enough to have access to a current clamp that showed my starter motor was drawing far too much current than it should do. It turned out that I had a short in the starter motor windings - measurements indicated that the short was in the field (case) coil and not the armature. When it was cold and the winding resistance was low, this was not so much of a problem and the starter motor would spin more or less OK. However, when it was hot and the winding wire resistance increased, the starter motor would struggle to get the engine past compression. Again, I was lucky enough to have access to an old Z900 which uses the same starter motor so I just "borrowed" the starter from the Z900 and this immediately solved the problem. Starter motors are fairly rare but a number of companies offer a re-furbished motor on an exchange basis - I live in the UK but had to get one form the US but it all worked out OK in the end.
There is no guarantee that this is also the issue with your symptoms but might be worth checking out.
