mosfet damage when chasis is connected to earth

i am using a STW26nm60 mosfet at the PFC stage its drain is connected to the pri_GND. The out put of mosfet is a 400v which will be driving the forward section and one capacitor section in which the capacitor is charged to 400 v in 1sec. all these section are in pri_GND and the out put of forward is sec_GND. The tranformer of forward is isolated.the complete unit is a 1200W power supply.The 1200W will be for first one second while charging the capacitor, once it is charged it is a 250w.The entire board is kept in a metal enclosure which is connected to the input earth
Now my problem when i turn on the unit with out capacitor, so load is 250W and I/P earth connected to the chasis the PFC mosfet is ok, but as soon as i connect the capacitor part the mosfet is damaged from drain to source. I did not found any other componenet damage and this problen is present in some boards
Can any one suggest me some clue reg what more i can check to solve the prob

can you post your schematic ?
I'm not sure I understand your description.
may be you have some stability problem, or it just overheats ?
What happens if you put 2 mosfets in parallel ?