I have five of these 877s on Zen VPN8000 connections with long lines on various BT exchanges. Two in particular with long lines are giving me similar problems with line drops. I deduced via Samknows that one exchange is 21CN and despite being upgraded to "20MB max" I can't get it to connect in ADSL2+. The ADSL firmware is 3.0.??, even on a replacement that I purchased a couple of weeks ago.
I'm now in the process of sourcing ADSL firmware and IOS updates via the Smartnet route so it will be interesting to see if it solves this particular issue. I've also ordered a consumer-grade Netgear DG834 (which from home experience works very well with ADSL2+) which will prove a point if it is more stable than my current Cisco firmware!
Update on this - I tried a Netgear DG834V4 on the 21CN line after having it set back to ADSL max It ran stable for a couple of weeks. I then replaced the 877 with another flashed with adsl_alc_20190_4.0.017.bin and c870-advsecurityk9mz.124-15.T16.bin - no further problems to date.
Repeated on the 20CN line - it ran OK until we had a few days of windy weather then it went very intermittent. One one occasion when the link was down I picked up the associated phone and as soon as I got dial tone the ADSL sprang back into life (line current bridging a dodgy joint?). Despite several fault reports and BT visits they have not found a problem with the line so BT (via Zen) suggested I try the latest adsl_alc_20190_0.0.026.bin (strange out of sequence numbering!). If the problem continues after this firmware change then they have said they will investigate further...