Fixing Punctures / Puncture Repair / Exploding Tubes

I can't believe that I am resorting to asking you lot about a puncture, but I am stymied.

I am suffering from a recurrent slow puncture that I just can't figure out.

A brand new rear tyre (Vredestein Fortezza Tricomp) went on a couple of weeks ago, before the Christmas nasty weather. The first time I rode it, it went down almost immediately and I realised that I had fitted the tube badly. Ever since then, I have had a slow puncture on the rear. I have used different tubes, tested the tubes under water for bubbles, checked the tyre multiple times for glass etc, nothing.

If I pump up to 120psi in the evening, I will have lost about 30psi by morning.

What am I missing? It's driving me mad.

Perhaps when you pump it up, air is trapped between the tube and the tyre, which gradually disperses so the tyre seems to be going down? I had this trouble with some giant dmr tyres on my big bmx...