I suspect a slightly different problem. Once you enter high speed level line of flight, there is a low pressure area that migrates to an area near the top rear of the canopy, where the canopy frame meets the seat back bulkhead. This is usually not an airtight seal. See the red area in the attached picture (low pressure).
When you are at lower speed and a higher angle of attack, this same low pressure area is closer to the front of the canopy where plex is a solid piece and is consequently airtight. I suspect that when you reach 140-ish the low pressure area serves to de-pressurize your cabin through the crack between the canopy and the bulkhead.
My thought is that the top of your fuel sight gauge (main tank) is in communication with the cabin pressure. If there is a small crack in the gauge tube inside the tank, then the low pressure in the cabin could draw air through the crack and into the standing fuel gauge column, showing up as bubbles. Regardless of the source of the bubbles (could be "boiling" fuel as previously suggested), such communication will tend to depressurize your tanks, and may be sufficient to overwhelm the ram air tube, especially if the ram is poorly positioned.
If I am correct, I think the only thing to do would be to install a fuel pump downstream of the header that can be used as needed to send pressurized fuel directly to the carb. The long term fix would be to install gauges that do not communicate with cabin pressure, so the ram air will be more effective.
My two cents.
Jay Scheevel, Tri-Q, still building.