'94 IDI surging, stalling, etc. Air in fuel?

[nixter]

Relatively new to the forum with a 1994 IDI Banks turbo that suddenly started surging & stalling. Something is telling me air in the fuel, but where?

The facts:

• Both tanks are well over 3/4 full
• Happened suddenly with no triggering event (IE fuel fill up, repair, etc)
• Activated the filter bleed valve and I am getting foamy fuel - IE there is air in the filter (key indicator to the root cause I believe)
• It's inconsistent and will run fine for a while then surge/buck or die when idling (not sudddenly - more of a 10 second loping idle then it dies)
• Changed out the filter/separator for a new FRAM unit
• Ran de-waterer through the system

Could my fuel selector valve be sucking air? Perhaps the lift pump going bad? Going to replace the lift pump as part of normal 'replace it before it dies' maintainence today and to eliminate that possibility

Any wisdom on the issue out there? MUCH appreciated!