Recurring problem with erratic download speed and poor ping

[#1]

I have a recurring problem - has been happening on and off over a number of months. At some times, my download speed is really poor and I get terrible ping times. It seems to be fairly random when this occurs. If I restart the router, then ping and download goes back to "normal" for about 2-3 mins, and then slows right down again. Then suddenly, at various times, all will be well again.
The router is a Belkin PlayF7D4402 v1 and is approx. 12 months old.
Here's a sample ping to bbc.co.uk at the moment - starts off relatively ok, then goes bad:
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=42ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=41ms TTL=55
Reply from 212.58.244.71: bytes=32 time=133ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1066ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1286ms TTL=55
Request timed out.
Reply from 212.58.244.71: bytes=32 time=1295ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1364ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1376ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1462ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1469ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1297ms TTL=55
Reply from 212.58.244.71: bytes=32 time=707ms TTL=55
Reply from 212.58.244.71: bytes=32 time=639ms TTL=55
Reply from 212.58.244.71: bytes=32 time=748ms TTL=55
Reply from 212.58.244.71: bytes=32 time=750ms TTL=55
Reply from 212.58.244.71: bytes=32 time=889ms TTL=55
Reply from 212.58.244.71: bytes=32 time=1034ms TTL=55
Reply from 212.58.244.71: bytes=32 time=830ms TTL=55

I ran the bt speedtester - again, erratic results ranging between awful (0.33,mps) and about normal (4.5-5mps)

[#2]

Do you use Spotify or torrent clients?
Spotify especially as it uploads large amounts of data and doesn't give a damn if it maxes your upstream.
Alex

[#3]

I don't use torrents, no - I do have Spotify but I shut it down when this happens and it makes no difference. Connection has been fine for a couple of days but I've been trying to watch iPlayer tonight and it's a buffer-fest - pings bad again:
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.
C:\Users\Admin>ping www.bbc.co.uk -t
Pinging www.bbc.net.uk [212.58.244.69] with 32 bytes of data:
Reply from 212.58.244.69: bytes=32 time=323ms TTL=55
Reply from 212.58.244.69: bytes=32 time=454ms TTL=55
Reply from 212.58.244.69: bytes=32 time=483ms TTL=55
Reply from 212.58.244.69: bytes=32 time=538ms TTL=55
Reply from 212.58.244.69: bytes=32 time=557ms TTL=55
Reply from 212.58.244.69: bytes=32 time=657ms TTL=55
Reply from 212.58.244.69: bytes=32 time=638ms TTL=55
Reply from 212.58.244.69: bytes=32 time=703ms TTL=55
Request timed out.
Reply from 212.58.244.69: bytes=32 time=634ms TTL=55
Reply from 212.58.244.69: bytes=32 time=697ms TTL=55
Reply from 212.58.244.69: bytes=32 time=703ms TTL=55
Reply from 212.58.244.69: bytes=32 time=737ms TTL=55
Reply from 212.58.244.69: bytes=32 time=714ms TTL=55
Reply from 212.58.244.69: bytes=32 time=374ms TTL=55
Reply from 212.58.244.69: bytes=32 time=409ms TTL=55
Reply from 212.58.244.69: bytes=32 time=420ms TTL=55
Reply from 212.58.244.69: bytes=32 time=531ms TTL=55
Reply from 212.58.244.69: bytes=32 time=574ms TTL=55
Ping statistics for 212.58.244.69:
    Packets: Sent = 19, Received = 18, Lost = 1 (5% loss),
Approximate round trip times in milli-seconds:
    Minimum = 323ms, Maximum = 737ms, Average = 563ms
Control-C
^C
C:\Users\Admin>