well first off welcome!
and hmm i really cant decide what it is, it doesnt have to do witha certain rpm, it is only in 3rd gear?
so it doesnt seem like and engine problem, bu ti tonly happens when its warm? hmmmmm
not to insult you , but your sure that you good at a clutch, its the only thing i can think of that would make a manual jumpo, buck and then go, if it kinda stalled as you put the clutch in