"Why won't the board let me post?" So read a PM to me, received this day.
I've had some troubles too.
From my experiences of late, it happens when there is a lot of activity on the forum or on the topic to which I'm accessing.
I'm on dialup - 50,666 at BEST, 40k typically.
Two errors that I usually encounter - can't find server and timing out.
Not much of an answer, just observations for others to add to the rest of their experiences and observations.