I am a late starter to biking and go from one problem to another! Now having probs with warming up my XJ600. She starts fine with the choke on, but when the revs build up I ease the choke in partly, and then the revs gradually drop for some reason. If I pull the choke back out then she dies, but if I turn it off she chugs and then stalls. When fully warmed up (eg after couple of miles) she runs fine and ticks over at about 1500 rpm. Can anyone help???

Sounds like the carbs need an overhaul. The choke mechanism is sticking and overfueling.