I had a similar problem last week with my Netgear DGND 4000 router - Talk Talk as ISP. My speed fell to about 3mbps from about 7, with frequent drop outs. Talk Talk claimed they tested the line the problem was my house wiring and tried the usual suggestion of plugging into test socket etc; they said they would send an engineer that they would pay for if their fault but I would pay £50 if mine. They claimed the phone line was fine - and I had no phone problems. This has happened twice before with them. I phoned PC World to see if they had a new router in stock; a nice lady in Hull said quite a few customers had bought new routers only to find the problem lay with Talk Talk!! I did not , luckily, buy a new one.
I escalated the issue to a supervisor and insisted he "re set" my exchange connection. Lo and behold, the speed instantly went up to 7.5mbps (My contract is 8) and I have had only one drop out since. I would insist that your ISP double checks the exchange connection, as Netgear are good routers - better than mos that the ISP's supply.