Power supply goes out of regulation at high input voltages

Hi --

We have a LNK603 design with 200 - 400VDC input and 12V, 5V, and 3.3V output. The system works well up to about 370V. Above this level, the supply starts to turn on for 11mS then off for 4mS. This results in 0.5V swings in the output voltage (we have 100uF on these.) I have tried various loading, changing the feedback resistors, and altering the clamp circuit without success. Does this symptom indicate a particular problem?

Thanks,

James

Additional Question

The datasheet indicates that Ifb can influence switch on time. Is it appropriate to reduce feedback resistors to lower on time? I've gone from 10K and 5K all the way down to 600 and 300 ohms (trying to eliminate the above problem.) Although I couldn't eliminate the problem, it was not clear from the datasheet whether this would be an appropriate way to decrease switch on time.