sometimes the front seat belts lock out and are not usable, obvious safety concern, other times they work as they should. Can't find any consistency in relation to any other condition...what triggers the belts to lock and how to fix this dangerous situation...?

What do you mean by lock out? They wont retract or they wont pull out from the retracted position.

I mean that they won't pull out from the retracted position, as if the mechanism thinks we are about to crash. It locks out usually in the fully retracted position and sometimes the belt can be pulled out part way then locks out...

I haven't had problems with that year's belts before. I have seen water get into the units & possibly yours had this problem & is now corroded. There is a weighted catch that pivots and locks the belt when the car moves quickly. It sounds like this catch is stuck in the lock position.