Wow, 158 users at one time! That's pretty good.
You get that error message when there are more than 15 users manipulating the forum (loading a page, posting a new message) at the same instant of time. Just sitting and reading a page doesn't count as "doing something". So, I guess it takes about 150+ people reading the forum to randomly get 15 people loading a page at the exact same second.
This is a policy limitation of the shared webserver and can only be "fixed" through the application of more money to the ISP's account. People have brought it to my attention several times last year, but because it's so random, and fixes itself almost instantly, it hasn't been a big issue to fix.
Please let me know if you get the error frequently enough to be annoying! (I assume we have peak hours and slow hours, so it might be interesting to know if this only happens on Wednesday nights from 7-9pm, or if it's happening every night for many hours).
Thanks,
Jeff