foul taste from stomach gases?

Periodically I wake up at 4am with this incredibly foul taste coming into my mouth, so foul it is almost unbearable and certainly prevents sleep.  It has happened maybe a dozen times in the past 3 years and when its bad it will last all day. It is not constant, will come and go between a few seconds and sometimes a few minutes (when its not that bad). Because its not constant it seems to be a gas coming from my stomach so it is a mix of a smell and a taste. I have tried to work out if it is a type of food eaten the night before, for a while i thought it was soft cheeses but I have it now and last night I ate garlic prawns with potato and salad, and 2 glasses white wine.  This is driving me crazy... help