After measuring the internet download rate (limited to 150kbits/s) I found out with firmware 1.9.2.7 I got sometimes only 20, 30 or 35 kbits/s. There is no problem with upload (my 70 kbits/s)! With firmware 1.9.5.0 I got 140-150kbits/s. Also the connection at startup is much faster.
The distance between WL-500g and WL-100g is in my case 2 floors. The signal quality in this case is on my notebook fair (35-38% in the Asus tool).
So if you have simular problems please upgrade to 1.9.5.0 and enjoy...