OK, I will look at water pump. I have no much hope because it seems not overheated at all. Oil is perfect, no coolant in the oil. Just changed the oil as well.Okay so just throwing my 2 cents in here...
My '79 CX would do that. Start great in the mornings, but once hot it'd never start. Couldn't even push start the bike. It'd drown the battery after a few cranks...even a new battery. Couldn't jump start it, either. I went through so much trying to sort it out and ultimately I just stopped turning the bike off unless I was REALLY sure I was done riding for a while. I dealt with it like that for over a year.
Then one day while riding I got a nasty puff out the back and no power. White "smoke", oil everywhere, chocolate milk in the oil, just generally nasty. Temp gauge never showed I was overheated. I went to change head gaskets only to find my headgaskets were both fine and unphased. Flushed the engine and she started right up and ran as good as she ever had. No exhaust gas in the coolant. Just real peachy. Except once hot...still...no starty. Totally miffed by this I dropped the engine back out and went snooping around. Someone here suggested that a bad seal in the water pump could be a potential source of coolant in the oil so I popped that cover off...and the impeller fell out. Splines were completely stripped, the inside of it was a mix of polished and tarnished and the hole was rather elongated. It had been that way for a very long time. After replacing a few seals and swapping a water pump over from another bike she ran perfectly...even started right up when hot.
The engine was getting too hot, causing things to start getting sticky. Coolant wasn't flowing so my gauge never really showed the temp spike in the cylinders and head.