Signal Strength, and Speed

[#1]

Hi I connect wirelessly. I used to use a USB network adapter which was OK but everytime I unplugged an I pod or other USB devide the connection with the router would be lost and would need to be repaired. I recently bought a pci card. Which has been OK. Windows shows the strength of the signal is usually very good or excellent with occasional lows. The speed however varies a lot. most of the time its around 11- 18 Mbps but can get up to 54 or down to 1Mbps when very occasionally  it fails to hold a connection. Quite often the low speed is at the same time as the signal strength appears to be "Excellent".
I've tried changing the channel ( from Auto to 1 to 11 and finally settle on 6)
Not sure what to try next. A better aerial on the card?
Without understanding the underlying problem its hard to work out how to make it more reliably strong and fast.

[#2]

welcome to the vagaries of radio - some call it a black art - and not a lot of people really understand it unless trained radio engineers and technicians.
But it may be interfering signals - even if you have been changing channels. Although the modulation scheme helps alleviate some problems, you can get multipath reflections ( signals bouncing off walls / ceilings etc )  which can degrade the signal quality. The "signal strength" display doesn't actually display real signal strength - it it the quality of the digital reception - the bit error rate.
Difficult to know what to recommend without knowing layout of premises and other details but an external antenna may help.