Aztek airbrush problem - unexplainable bubbles in the paint cup

I have a problem with my Testors Aztek A470M double action metal airbrush. It is still under warranty (about six months old), however I thought that if it is only a minor problem it might be easier for me to fix it myself rather than return it to the US for repair (I am in Australia). I have had great use out of the airbrush over the past months, however there now seems to be a problem (the problem started a few weeks ago and has got progressively worse to the point that I can longer use the airbrush). I have been using acrylic paints only (mostly Tamiya brand). The problem is that I am not getting paint flow through the nozzle, rather I get lots of bubbles in the paint cup. I am a fairly experienced modeller, so I have tried a number of the usual things to remedy the situation, with no success. Normally this problem (of back flushing into the paint cup) would indicate a blocked nozzle, but this is definitely not the case. The airbrush has been thoroughly cleaned and internally inspected by me. I have tried using a number of different nozzles (all clean) and none of them have fixed the problem. Using a high flow or low flow nozzle does not seem to make any difference. I have even bought a brand new nozzle to make sure, but had no more success. I can confirm that compressed air passes through the nozzle without problem (when there is no paint cup and the opening is closed off), indicating that there is clear air passage all the way from the compressor to the nozzle tip. I have thinned the paint correctly so the problem is not caused by thickened paint (even water and pure thinners will not go through). I have tested to make sure that water can pass through the airbrush from the paint cup out through the front of the airbrush (where the nozzle would screw in) - there appears to be no blockage there. I have tried a range of pressure settings to ensure that the problem is not related to a lack of air pressure. The problem seems to be that while air can pass through the system, the paint is not being sucked down from the cup (this is true for all cups and jars). I have ensured that the breather holes in the cups are free (and I have even tried the cups with no lid to make sure). Logically, the air is going to follow the path of least resistance, which in my case is up through the paint cup rather than out through the nozzle. I have tried different nozzle depths by loosening or tightening the nozzle with no success. During my fault finding expedition I found that if I pressurise the cup (I used my mouth to gently blow water through the system from the paint cup) while simultaneously releasing air by pressing the trigger, the water did come out of the nozzle, again proving that there is a clear passage between the paint cup and the nozzle tip. I have tried submersing the airbrush underwater to check for air leaks, and there is a very minor air bleed that comes out of the black plastic head at the seam. Any advice would be appreciated.