Why would a TNY28x cycle on and off at 300 mS period?

... and is there really not a seach for the forum?

I have a TNY288 based flyback which mostly works but in certain circumstances will cycle every 300 mS. It runs for 70 mS (which might be the nominal 64 mS) and then shuts down for 230 ms. The data sheet tells me what to look for if it cycles every 2.5 seconds but I can't fin a mention of 300mS. If I overload the output, I do get the 2.5 second behavior. But what condition would cause cycling at 300 mS? I don't think it's low-line because I temporarily reduced my low-line sense resistor from 4M to 2M and it didn't seem to help. The link supply is full-wave rectified 120 VAC 60 Hz and shows 165V on my DVM.