RunningMan: How inclined are you get your hands dirty with checking the wiring yourself? If you can figure out where the leadin comes in from the street (either aerial or underground), you should come across a master splitter fairly easily if one is fitted. You can then temporarily connect the modem and see what happens.
Jase2985: get something like DSLstats if it works with your device and get it to record the modem stats and graphs for you.
RunningMan: Given the huge drop in sync rate at about the same time every day, I'd guess some sort of external noise feeding into the line - Some large electrical appliance nearby starting up?
To narrow it down, finding the pattern of when it occur is the key, as it could be time, temperature, moisture, movement related...
I'll probably reach out to Spark today to see if I can get this more formally investigated. But I'll keep the logging in play so I can continue to build up an idea of what exactly is going on here WRT sync speeds.
RouterStats showed that overnight our connection went down around 12:50AM, and stayed down until 12:57. Prior to going down, it was synching at 16Mbps/1Mbps. When it came back up, it was syncing at 379Kbps/888Kbps. It is still syncing at this speed.
I've hit the ball back in Sparks court, and will see what they can do from here.