Well you said in your heats you had no problems, so your needles should be ok. How long is your exhaust tubing? If it was my car I would at least test out a new tank but that's because I use Mugen tanks...

Your idle could be dropping when your engine warms up, but you would also notice this in a 7min heat. So I would rule that out.

For me it really points to a tank or line issue.