Right this is doing my ****ing head in..

Basically everytime about 15mins or longer into my mix. One channel, lowers and only comes through 1 speaker @ the proper volume..

So the result when listening to it back, is the tune am mixing in coming thru more predominantley on the right hand side lets say...

I can tell when its gonna happen coz my headphones only plays on one side when cueing mix..

Ive reversed the in/out cables on each deck, and then the problem reverses to the other deck..... I cant do a decent set, as this is always happening..

Am thinking its a cable @ the back but there only a year old and are expensive ones.. But am really hoping its not my mixer which is a Gemini FX7000.... My decks are stantons...

WTF is happening. Am thinking its my cable, but why?

Anyone had this before..