The only time my Yaesu radios do not transmit a signal but appear to key up is if I am out of band. Even one Hertz out of band will cause the rig to not transmit. Not saying this is what you're doing. Just an observation on my end. It wouldn't make sense though, that cycling the radio on and off, a couple of times would put the rig back in band, or for that matter, repair your tuner. What you're describing is very mysterious. Spooky tuning. I would explore if the rig is possibly locking up from some bad juju flowing back down the coax from the G5RV, and only occuring on 75/80 meters. Heaven knows, when I force RF into my G5RV on 160 through my tuner, weirdness happens in the house sometimes.