My wife has asked me to post this - she has been following Trev's plight with some concern. Anyway, her Mum used to have a GSD that went like this every time there was a bitch in heat on the street. Sometimes, he'd be like it for weeks on end, then suddenly right as rain. They did loads of vet visits - nothing. It was only when it kept happening that they realised what was going on.
We're not dog experts, but hope this might help, and hope Trev gets better soon.