Aux input drops out randomly when connected

[LukeGI]

I installed the Aux input cable on my 2004 X5, without dsp, without navigation.
I then proceeded to connect my iphone to test it out. It played fine...for a while anyway.

The first time I use it, the stereo will stay on the aux input mode and play from the iphone perfectly, and then at some point it will drop and switch to the radio. Pressing "mode" will not get the aux input back. If I disconnect the iphone, I can then go back to the aux input and play the iphone again, but this time for only a minute or so max before it goes back to radio and Aux is no longer available.

If I simply leave the cable unattached to the iphone, the radio will stay on Aux indefinitely...no issue whatsoever.

Any suggestions on how I get the iphone to play? I have tried an iphone 4 and 5, but have not located some other device to test it with besides the iphones.

Thanks!