'Hunting' when hot

[R_I_C_H_A_R_D]

Hi

I have just bought a 2001 TXT249. After about 15 or 20 mins of puttering around and the bike is nice an warm, the revs seem to climb and fall, almost as though you are giving it little one second bursts of an eighth of a turn on the throttle. It gets so bad that the bike actually cuts out in the 'dips', and if you give it some throttle as the revs fall, the revs pick up pretty slowly. I am running 100ml oil to 5l petrol - Is this correct? (50:1)? I am not sure if this is related, but even when the bike is hot, I still need to use the choke to get it started. I am new to two strokes, but as I type this I think perhaps I should have replaced the plug as a starting measure, but think possibly it is an air screw setting or something else in the carb - but as I said I am a novice so any (simple)advise would be appreciated.