I'd go through the clutch plate by plate and debur everything with a stone or jewler's file. My '03 was a bitch when new and this fixed it. Also, I wonder if the cush drive blocks on the older bikes like Stainless's could be worn, causing unwanted movement in the basket in relation to the center hub/pressure plate, especially when the bike is stalled under load. This would not explain the problem on a new bike though.

A few extra small holes in the pressure plate like shown on the KTM mod can't hurt, but I wouldn't go crazy.

I have the same probs with my ec300 '06 when i bought it. I pulled the clutch apart, three of the steel plates were warped, new steels and clutch is now good. You need a good flat face plate to check them on. More than 0.05mm of warp and they should be renewed.;)
Reckon that this is why it gets worse when hot, as the steels expand and increases the warpiness.

when i replaced the steel plates, i checked new ones with feeler gauge/sheet of glass - they were good.. i may try the ktm clutch mod to see if that fixes it.

with my 200 I found that the knotches in the inner basket were enough to cause an issue. Once I painstakingly dremeled smooth it would kick in gear again. At least for another year.

I took a 9.5 magura, hooked it up then took the slave cylinder off the engine, put a spring loaded clamp on it then pulled in the clutch. You get more movement of the piston in the slave cylinder compared to stock however you can just start to see the first o-ring. A clutch failure on a mntn goat trail 90 miles from no where convinced me to not use it.

Try using Castrol MXT oil. I went through all different oils and this seems to be the best when clutch drag is concerned. I've been running this MXT for the last year and when the clutch starts dragging it means its time to change the oil.