Can someone advise on the best WAN detection method, so that failover can kick in when the primary internet connection is not exactly dead, just very slow and flaky?

I live at the extreme end of ADSL reach, with 2 ADSL modems going into a 2930 router. Quite often the S/N ratio of the primary connection drops to 0DB and the connection effectively stalls for a bit, though I can still get a ping through. I'd like it to trip over to WAN2 which could be doing better, but it stays on WAN1 because its still just about alive.