spawning starts then stops

[fivespeed302]

I have a pair of 3 yr old FH's that are always in spawning coloration, always digging in the gravel, cleaning rocks, chasing each other, etc. They will spawn 4 or 5 times, about 5 weeks apart, raise their fry, and then they just stop for months. I have been keeping track of their spawns and there doesn't seem to be any pattern to it, so I don't suspect it to be a seasonal thing. They still have all the spawning traits, but the female just won't lay eggs with regularity. One time the female got really beat up so I put her in another tank to recover. When I put her back, she spawned within a week. Is separation the key for spawning? Any comments would be appreciated