sony das 702es dac problems..

hi all
ive juse aquired a sony das 702es dac.
the trouble is it has some problems in the form of noise.
its fine when first switched on,then after about a hour and 40 minutes it develops a noise similar to static from a radio on the left channel when playing music,if i stop the music playback then the static stops through the left channel but is replaced by a loudish wooshing noise through both channels this time??
anyone have any ideas,clues where to start?,i was going to recap the dac but that will cost quite abit,i dont mind doing it as the breif listern i had of it seemed quite decent,however i dont want to invest a small fortune if its a problem i cant sort out,also opinions regarding the quality of this dac soundwise would also be appricated compared to todays offerings for those that have owned or heard them.
many thanks in advance for any advice/help.
all the best