Strange air ingestion problem on the '98.

[KarTek]

I'm continuing to work to get my car running right and it's down to air in the fuel lines.

Here's the strange part: The fuel lines will be completely air free then, suddenly, the fuel just drains back out of the line leading from the fuel heater to the pre-filter. Completely empties! No fuel flows for about 4 seconds then the engine starts running rough from lack of fuel. Then, just as suddenly, the fuel rushes back into the pre-filter, the engine smoothes out and all the lines are air free again.

The process repeats itself every 30 seconds or so just like clockwork and is not effected by the speed of the engine.

Is there a tank screen or something else in this car that would clog, cause a cavitation and then release?

Any other ideas what's causing this? It never did it before I replaced all the o-rings.

Thanks!