Sudden wireless connection problems

In last few days I cannot completely load some web pages e.g. ebay, moneysaving expert - when connecting wirelessly. All is fine with ethernet.

ISP is Kencomp - microwave wireless (10 Mbps) to an Edimax BR-6428ns router.
Equipment: Imac, macbook and Iphone (this suddenly not connecting apps that need wifi).

I distribute the network with homeplugs (one wireless) and all has working flawlessly for 10 months. Now all of a sudden - and without any changes to the equipment - I am having the problem.

Kencomp suggest I the Iphone maybe the culprit but I have ruled out this by switching it off. I have also disconnected the Homeplugs.

I have accessed the Edimax router and changed the channels one at a time with no change.

Please has anyone any ideas?

Depending on your OS - but check your MTU settings both PC and Router. On XP you can adjust, but Vista and newer it is "auto".