OK, sorry, we both missed what you meant by "gtx 285 SLI". We assumed you had only one 285, and wondered when you installed the second one.
Does removing the second 285 lower the temperatures significantly? Probably not, I'd guess. If not, and if the heatsink isn't detached, I can't think of a reason why this board running at stock (make certain it is) would suddenly heat up unless the mobo's got a problem.