802.11n WiFi sharing 3G broadband - sometimes won't connect to the 'net

mostly things work fine but, every now & then, we find one or more of our computers can't connect to the Internet ...

they're interconnected via WiFi and, when this happens, repeated attempts to get things working may or may not be successful - it seems random - even after many many attempts ...

sometimes it's just one that can't, sometimes it's two, sometime's it's the host as well !

iMac 10.6.4 / Mac Book Pro 10.5.8 / MacBook Air 10.5.8 / iPad / and sometimes iPod touch & 2xiPhone 3Gs too ...

usually we host the WiFi from the MacBook Pro, sometimes from the MacBook Air - because doing it from the iMac was never successful after updating it to 10.6.4 ...

but also because our Mobile Broadband 3G Stick won't work with 10.6.4 and at some point in the past we found we needed to have it on the machine that was hosting the WiFi (still true or not, don't know) ...

and, when we set-up a new WiFi network (via System Preferences / Network) on say the MacBook Pro, the message that appears under the Status: On indicator seems randomly to say that we can, or we may-not be able to, or we cannot, connect to the Internet ...

and when we then have the iMac & the MacBook Air join the network, the message they display seems similarly random -

OK, not quite, because if the host can't, they can't, but if the host can, they may or may not ...

when they says may-not, it is indeed the case that sometimes they can & sometimes cannot ...

when the battery dies in the host, or we put it to sleep (to go to bed, dinner, out, whatever) the network goes down, of course, and the chance of having difficulties (as above) when we start-up again seems to get greater ...

we're perhaps unusual, in that we're needing to re-create the network once or twice every day ...

and the problem occurs even when we're careful to turn Airport off (via System Preferences / Network) on the all three computers - MacBook Pro & iMac & the 'Air - before creating anew & having them join in-turn ...

we did buy a Time Capsule, to be able to have a permanent network - but it wouldn't work with 3G mobile broadband (we don't have a land-line), meaning we wasted our money !

I have a suspicion the problem could lie in lags - ie: the nature of WiFi has led the AirPort developers to include code that ensures some persistence in hanging-on to a network, ability to remember WiFi network details, even after they're gone, even when they've been gone for a while ...

so - is there something we're doing wrong, or is there a way we can command things to be, as it were, re-zeroed, so we can start creating a network from a clean sheet ??

any thoughts, please ??