My experience has been that if there's *any* air leak between the tank and the pump, the pump just flat quits. I've seen this various times with various specific failures. I don't think you're drawing air into the pump.

I've also seen lots of bubbling in clear fuel filters, which hasn't hurt a thing. I don't know what causes it, but I say it's not contributing to your problem.

If fuel flow is inadequate with SU carbs, the symptom is a complete loss of power on two or four cylinders as one or both float bowls run dry. A momentary drop in fuel level, short of the amount that kills combustion entirely, has very little affect. I've run such problems at great length (bad pumps, line blockages, needle valve problems) and have never experienced lean surging as a result -- it's more an on-off thing.

My guess is you've either got an ignition problem under high-rpm load, or carbs going unbalanced under those conditions. Neither is particularly uncommon.