That is wierd that it would happened in 1 FS session (no scenery library update). Does ai re-appear if you fly a different ac?
Though I've never had or heard of this specific problem, here are a few thoughts.
Did you check your ai sliders?
Did either of the ac install scenery (that may have included ai traffic files)? If so, you might want to check your entire FSX directory (or remote scenery folders) for FS9 traffic files. AI Flight Planner can do that. http://members.shaw.ca/aifp/
Have you tried un-installing the 'problem' ac?
Out of curiosity, what are the 2 payware aircraft?
At that time  the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.