ZOA Controller described a developing conflict event when one of two military aircraft operating as a flight elected to depart the assigned altitude.

Narrative

I was on the R-Side. ZOA APREQ'ed a F18 flight at FL370; I approved it and requested control. When the aircraft came up on my frequency; I turned the flight for traffic. I then scanned for other traffic and realized that I was going to have to turn the flight several more times in order to get them through my airspace safely. I offered the flight FL430 to avoid all of the turns. They accepted and I climbed them. The training team then came back and I was instructed to move to the D-Side. Shortly after the training team took over; the flight requested lower due to aircraft performance issues. The R-Side issued FL400 and asked if the flight could hold that altitude for a few minutes for traffic. The flight lead said yes. A few minutes later the wingman called and requested a code because he was separated from the flight lead and was at FL365. The instructor took over the position and noticed the primary target that was most likely the aircraft in question. He instructed the aircraft to descend and turn 30 degrees left to avoid traffic that was head on and only about 15 miles at FL360. Eventually the aircraft was given a code and a separate flight plan. Weather may have been a factor in the loss of the flight lead; but I doubt it; it is a mostly clear day. Recommendation; pilot needs to be questioned about why he descended below assigned altitude.