For your LSAPI config, having "Max Connections" and "Instance" values of "500" is overkill if most of your php scripts connects to a db.
To best judge what you ready need, check real-time stats during the peak hours of your forum and see how many are really in use.
More php processes does not equal to faster website because at the end, 500 concurrent php processes would need 500 concurrent mysql threads which is a load killer in it's own right. We have done some benchmarks and lsapi php values of 20 can be enough for a lot of websites provided the code is clean and mysql is humming along.
Try Max connections and instance value of "60" which should be more than enough.
As for the problem in question of 503 errors or temp busy, that's definitely a problem but appears unrelated to the config in quesiton, at least when I check the throttle values which shouldn't kick into effect.
Have you been able to reproduce the problem yourself?
The only thing I can think of right now based on info is that the throttle is kicking in. Try this. Find the forum user that is having this problem, have them reproduce it on the spot, than set the throttle values to 0/disable, restart lsws, then ask them to reproduce the problem once more.