Every so often, usually at least once a day, some post will just not open for some period of time, despite trying over and over again.

It appears to be random in terms of the Board or feature.  (For example, this AM it is the Top 10 poster Board, 6 month view.)

This has been going on for about a year, and started right around when the New TER came out.  It happens on both the New and Classic TERs.

Can this be fixed for once and for all?

It is random as far as when and where it happens. I usually clear the cache when it happens and that helps sometimes but other times it continues.