Sounds like the same problem i had with mine my a/c bike last weekend which has an adjustable clutch,my clutch was overheating and it would just slip,i left it for a few minutes to cool down and rode it and it went hard again
If its reving and not picking up like it did sounds like a overheating clutch to me mate,btw it doesnt have to be a hot day to overheat a clutch,if youve got it adjusted to kick in at higher rpm theres a good chance it will overheat easier reguardless of the outsside temps,try backing the springs off a little so it engages a little earlier and see how it goes
