Hi Steve,
when your download speed deteriorates, the problem can lie anywhere down the path from the source site to your PC. The trick is to identify where!
In general, the speed of Cable broadband isn't as sensitive to load on your ISP's network as ADSL down the phone line but the trunks are still shared and that can cause some variation. From your description, it sounds like you are seeing a degradation, compared to when you forst got the broadband. This can be somewhat subjective, though, so it is worth running a speed test to check just what speed you are getting. CNET/Techrepublic/ZDNet have speed testers and these work reasonably well. It would be nice, of course, to compare with what you had originally but that is water under the bridge. So run the test every hour throughout the day (or at least that part of it you are awake!) and see if there is any significant variation, which would imply that you are experiencing load sensitivity.
The most common cause of load sensitivity on a cable connection is transitory overloads on the ISP servers. Here in the UK NTL (now Virgin Media) used to add a server when the average load reached 70% but other ISPs will use a different threshold. If this is what you are seeing, it is worth calling your ISP to discuss possible causes. Your ISP may also have a speed test specific to their network that they can give you.
I'm not sure what the activity light on your modem is - do you just have one or do you have separate transmit and receive lights? I'll assume it's a receive light, which would relate to your download sluggishness. It should only be on when data is actually being transmitted. If it is more or less continuously, that suggests there is a lot of data downloading. If this is more than you would expect from what you are doing, it could be error correction requesting retransmissions. If so, it could be due to deterioration of the cable itself, or the modem reaching end of life - you say you've had it for some years. I'm assuming here that our ISP hasn't increased the bandwidth on your line - if it has, it may be too fast for your modem and it's overrunning and requesting retransmission. Again, this is a call to your ISP. They should have diagnostics they can run to your modem to see if you are getting heavy error correction and if so, where. If it's the cable, expect a long wait while they replace it. If it's the modem, that's a relatively quicker solution.
Since you have your PC and your son's PC (albeit powered off) online, I assume you have a router in your home network somewhere. If the router is connected directly to the modem, before calling your ISP, it would be worth connecting your PC directly to the modem to see if that cures the problem. If it does, then the fault is on your internal home network; if it doesn't, then it's most likely the modem or the cable - or the repeater boxes on the line, again, a call to your ISP.
That's my "starter for 10" - hope it helps!