...So 75 280z, L28E, all stock.
Starts up fine from cold, proceeds to warm up well, idling/running smoothly...
The problem comes when I switch the car off, wait about 15 or so minutes and switch it back on... For about one minute fully, and generally for no longer, I get rough idle/revving and missing, after that point the car smooths out with no further issue
If I wait only say, five minutes after shutting the car off (filling up at a gas station, for example), this problem does not occur, yet if I stop to get groceries, by the time I get back to the car it's rough. By the time the engine is cool again (overnight), it starts up fine.
...So the obvious answer would seem to be an injector leaking fuel into the intake while the car was off, but then I'd assume it would stay pooled in the intake and still affect the car once it was cold. I don't feel like the extra air valve or any part of the cold start injector system could be the issue, as they shouldn't cause the car to run so roughly in any case, should they?
I haven't bothered to junk the EGR valve yet, but if it's stuck open, would those symptoms be possible? I'm almost certain I can hear it hissing from it when the car is at idle, which I get the impression should never be something that happens and could be indicative of it sticking open at idle (though I may be hearing things at this point)
I feel like I've eliminated any other source of possible vacuum leaks, which in any case wouldn't/shouldn't be so temperature dependant... Right?
Does it sound like I'm on the ball with this, or does anyone else have any more reasonable explanation for my problems?