a) I think you have to change the router's settings, as it's now connecting to a home 200 modem, whereas before it was a home 150 modem.
b) Do you still have the manual?
c) Hindsight is a wonderful thing, but it sounds like your router is shit. I had the same problem a while back. Saving an extra £10-£20 isn't worth it when you end up with a piece of crap that doesn't work very well. Ideally, you should have got a router with an internal modem so you wouldn't have all these problems now
Have you tried changing the channel it operates from? This sometimes makes a difference. I think the default is 11.