Been having troubles with Cisco 870 (877) routers with recent IOS revisions.
Earlier revisions had lots of bugs, some nasty and some cosmetic but the real need for a recent release is the improvement in SIP ALG and the addition of several features. Unfortunately the newer releases aren't much better.
Seeing serious issues with 12.4(22)T where after a month or so the router runs out of contiguous memory and stops. It doesn't reload - it just becomes incapacitated, which is much worse than a reload. Syslogs show increasing memory fragmentation (rather than a leak per-se).
12.4(24)T is worse - it reloads every half-hour. Haven't been able to grab the console debug yet.
Another bloke has blogged similar issues - I've pinged it here http://www.alcatron.net/wp-trackback.php?p=154

Update:
c870-advipservicesk9-mz.124-24.T1 is stable, both on NAT and SIP ALG.