Mountain Lion WiFi

Hi all, hope you can help.

After updating to Mountain Lion, I've had WiFi drop issues - common it seems - but these seem to have escalated to the point where Finder & WiFi can't live together so at times, the computer is virtually unusable.

If I try to open a file or folder when WiFi is on, the beach ball appears and it takes a minute or two fo r the file/folder to open.

With WiFi off, files & folders open as they should. In safe mode, WiFi & Finder work ok together, although the whole computer runs a little laggy.

It seems a little better on a separate really slow wifi network we have, but that network is a little too slow to use.

I bought a new router in case that was the issue but it wasnt. Other devices & PCs in the office work fine on the same network.

I'm not technical ⚠ - I use the iMac for business & it's causing me some real issues.

Anyone here seen such a WiFi / Finder issue & resolved it?

I've a 2.5 GHz Intel Core i5, 12GB 1333 MHz DDR3 iMac running 10.8.2

Many thanks in advance for your expertise,

Haider