Reply to thread

It sounds like you are saying that the Vin supply to the UC3842 is not stable?......where is it coming from?, a bias coil, a high voltage linear regulator, or somewhere else?

50k also sounds very high value for the timing resistor in uc3842.

Why not get an equivalent circuit  going in LTspice then send it to us here....LT1243 and family are the saem pinout and function as uc 3842, so you can use that. Most probably, when you make out the simulation, you will realise what is the problem.