I don't have any first hand experience with the 871, but from the description of your problem, what I'm wondering is whether the 871 is just being crushed by bandwidth of the large download. Do you have any recorded performance stats before the stoppage? What's the maximum DSL downlink bandwidth? What version and feature set are you using?
Perhaps one way to easily confirm whether it's the bandwidth push causing the issue, shape the outbound rate on your 7200 to one Mbps and see if the problem still occurs. If not, start to ramp up until it breaks again.
If it is bandwidth related, try to optimize every aspect the impact of the configuration options of your 871s. This might include items such as trying, if supported, AES instead of 3DES, reordering ACL sequences (most hit first), removing IP accounting, removing ACL logging, buffer tuning, insuring debug mode is off, console logging off, using longer load-interval, changing scheduler ratios, insure all packets are not being fragmented within your GRE/IPSec tunnels, etc.
Running the most current patch IOS release might preclude you from memory leaks and other issues that might only arise under extreme stress.
