Internet dropping to such a slow speed even Google won't open

[#1]

It's fine for five minutes, usual speeds of 1.4MB/s and then bam, nothing opens and everything keeps timing out.  Reset the modem/router and it's the same, five minutes of normalcy and then no speed at all.  Have tried two different modem/routers and both give the same result.  The phone line is quiet, with no noise.

Have had no problems leading up to this, I just turned the PC on today and this is what's happening.

[#2]

I don't know what happened, but I ran through the fault process and then about a minute after I'd finished going through all the steps and submitted the report the DSL dropped for a minute, came back on and now it seems to be back to normal.