Yeah, the garage. What would the symptoms be to your "immediate knowledge" then?
The symptoms you've described (with respect, as clearly as you can describe them on a forum) are all pretty non-specific. What I mean by this is headaches, dizzyness, palpitations have any number of causes for example. As for other things you've described, e.g. tea making your mouth turn the same colour for hours, ok this might seem specific to you, but just sounds odd. You're implying that your saliva turns the colour of tea. I'm not here to give medical advice as this is not allowed, and would be irresponsible of me.
I'm not just jumping on you smoking because it's an easy target, but it is common sense that smoking can cause oral problems. I know you said your dentist said it wasn't an 'oral' problem, but with respect, all this could mean is that there is nothing wrong with your teeth and gums.
Everything i've looked up even close to what i'm experiencing lists other stuff aswell, very serious stuff and it gets me all depressed and crappy reading about it so i try not too. At the very least i'd have expected the doctors to take a look inside my mouth, the colour of my tongue etc - but no. Get my sinuses checked out, my digestive system, anything! The only thing they seem to focus on after all my moaning is my chest which is probably the only thing thats got a bit better since leaving the room.
Easy way to get a doctor to examine your mouth and tongue- just open your mouth and stick out your tongue. I'm being serious. If your tongue was a worrying colour and you stuck out your tongue, no doctor would ignore that.
Going back to the dentist thing- I presume your dentist has looked inside your mouth and seen your tongue (wouldn't be much of a dentist if he didn't

) If he said it wasn't an 'oral' thing but acknowledged there was something wrong, then he should be able to write to your GP to formally describe his findings and request further investigation. I presume your dentist didn't feel this was necessary?
Getting your sinuses 'checked out' isn't as simple as you might think, especially if it might not be your sinuses at all and only what you believe to be a sinus problem. Similarly, getting your digestive system checked out is probably not as straightforward as you think.