Yes, I am guessing that the cause for transfer failures of large F@H/BOINC result files is (or has been) bad TCP/IP packet queuing by the cable modem-router or a router at the ISP, coupled with upstream routers of 3rd parties eventually not tolerating the huge latencies when the routes are respectively long.
Physical layer problems or maybe just plain overbooking manifested as my other problem: Actual connection losses between cable modem and whatever sits upstream, which sometimes didn't happen for months and sometimes happened several times a day especially on weekends — most of the time followed by automatic reconnect, sometimes (generally at most inconvenient times) followed by the modem just doing nothing about it until I reset it.
Anyhow. Now that the required ducts have been laid in our street, I hope that fiber is threaded in sooner than later, and that the apartments in our house finally get FTTH...