Cannot find Airport Express - wirelessly

Hello everybody,

I have been using an Airport Express to great satisfaction until recently. I have now different modem and router because I have new provider. I also installed Leopard on my MBP. Since then no luck in getting my Airport Express to work fine.

I have now: Ubee modem - Sitecom Router - AE (for streaming music only). I have changed almost any item in the router and the AE that I can think of. The AE is in bridge mode, has the same WPA2 key, same channel but not that of the neighbours, etc etc

8 out of 10 times, the airport config program doens't detect my AE at all not even after resetting it over and over again. If I connect my MBP and AE with cable - no issues and I can configure the AE to exist my current wireless network. But as soon as I unhook the cable - no AE although the led on the AE shows green continuously.

1 ot out of 10 times, after several hard and soft resets I can configure the AE wirelessly but when I want to round off the proces, I receive the message" Cannot find the AE".. grrrrr

1 out of 10 the configuration works, I can stream music for abut 2 hours and then I lose connection and the AE is gone (although solid green light).

It makes my head spin! I have talked to sitecom for hours, with an apple commercial helpdesk, browsed the web but nobody seems to know the answer to my problem.

So now this is my final call - HELP!!

All my other products connect fine...