Have you contact broadband provider on the 871 side to rule out any issues on the link? what broadband ADSLAM pppoa? start first rulling out physical issues WAN interface, LAN interface stats and work your way up, is this is something that suddenly developped? from what you post indicates it seems this tunnel have been fine, it could be broadband link issues but fist investigate with provider to go the next step.
what do you see in 871 router logs in terms of links, turn on logging informational before staring debugg proceedures.
HTH
Jorge
Jorge Rodriguez