Did you rebuild bottom end ? And was crank replaced or checked ?
All seals replaced?
Did you run it in per rotax specs ( takes an hour) and note all temps etc...
Also just as important as correct warm up is the cool down .
Here's my 503 -run in specs...( no cht on the nieuport )
Also the same for my 582...
I have been a 2stroke mechanic( mainly moto x works machines) for 26 years now, never come across an intermittent RPM rise of that calibre due to outside temp?...Have had loss of RPM intermittent .. Turned out dad and son mixed fuel different, dad added extra100 mls to jerry can making a strong rich mix, lol.
What oil do you use and have you changed oil brands?
Did you fit piston per specs or by replacing it as it came out of engine? I ask as I had an apprentice once who removed a piston and refitted by noting the old piston position , however the old piston was backwards ( arrow pointing back) yes it made another mess of the customers kx 250 requiring another full top end replacement including the kawasaki power valve. Grrrr cost me $$$$$
What spark plugs are you using?
No worries
The More info the better :)
Good luck