|But how could 2 routers generate the same problem?|
It isin't likely that two different routers would generate the same problem. This is indicative of a network interface problem on the device you're connecting to the WLAN with.
Troubleshoot the wireless interface. First, check for updated drivers, if you find some, install them. If the problem goes away you're done. If it doesn't, you might try uninistalling the TCP/IP stack for the interface, reboot, then reinstall it. If the issue still doesn't go away, the hardware may be broken.
I have a download/upload speed graphing program, and whenever I download a file the graph looks very odd. It spikes for a few seconds then goes way down to 0 for a second then back up to full speed.
I wouldn't worry about this too much. First of all, if your wireless interface is having issues, that would stop once it's fixed or replaced. Second, if you're downloading from a busy server, or the network itself is very busy, you could see a flow like this.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.