Late reply!
I really doesn't feel like it is technique oriented. It feels different.
I've done tours for bout 20 dates in a row when my voice been fine throughout the whole tour, even improving.
Then suddenly, my voice can change almost over the night on an off-tour/vacation period.
As I mentioned above. The reflux thing is probably not the issue. I've been scoped by ENT during bad periods (singing wise) and my folds have been in good shape without any redness och swellings. So that must rule this darn GERD/LPR thing out.
So it must be something else. Coming on so suddenly.
Two theories I have:
MTD
Neck, shoulder tension - Sure, I feel tense on the off days. at least the production feels tense.
But it doesn't ever hurt, which I believe MTD can do? Phlegm
Could it be as simple as this? When I'm singing on a rough day, it can almost feel like my nose is a bit stuffed and that something is stuck in my throat, although it doesn't come off.
Some days it has even turned around like being really out of control at noon and great in the evening (very rare though).