wireless connection behaves differently to ethernet, same iMac, same router

New 21.5 iMac, HP 5510 wireless printer, WD MyBook Live drive connected to a 6 month old BT Home Hub 3 router.

The new iMac joined my stable wireless home network to replace a Dell and join 2 x MBPs/Lion 10.7.2, 2 ipads, 4 iphones and a new Windows 7 laptop.

Actually this is the second iMac this week, first one went back, the superdrive didnt work, but I had it long enough to see the same wireless behaviour, frequently dropping the signal requiring a wireless on/off to get it back. I've seen the traffic in the forums, mainly slamming Lion for this. However, whilst both iMac's I've had did the same, my MBP's running Lion have never missed a beat.

I can just about tolerate this because I always intended to have a wired connection to the router from the iMac, but it has dented my faith for now. In trying to improve things I found nothing helped on the iMac side, but experimenting with different channels on the router and setting the security to be WPA2 only, the drop-outs are reduced to an irritaing and unpredictable few times a day.

What has really perplexed me is apparent differences in behaviour depending on whether I have wireless switched on or off on the iMac. It has taken me out of my envelope of knowledge to understand what the **** is going on. I don't want wireless switched on, I want wired for reliability, but I have a ridiculous situation where I get more functionality out of my network if the iMac has wireless switched on as well as being wired to the router.

Scenario 1

WD MyBook Live ethernet to the router and set up with the intention of using it as a time machine for the iMac and a shared drive for MPBs and Windows 7 laptop. All seemed fine with the wired connection, fnder and the browser can see it, BUT Time Machine will only see the WD MyBook Live if wireless is switched on.

Scenario 2

HP wireless printer set up and software installed on the iMac. With wired only, printing is fine but scanning doesnt work, the scanner and iMac cant see each other for scanning. With wireless switched on scanning is fine as well as printing.

I just dont get this, I haven't wanted to mess with it all any more, Ive spent hours going through the HP software, the WD software, the router as well as the network preferences and it just doesnt make sense. Anybody who feels they know what's going on here would be a genius in my book.