Not knowing if the Intel card is based on the Atheros chipset, but the symptoms you describe, sounds like the ones
Marius wrote about regarding the Atheros support in Netstumbler.
As why you have the same problem outside of Netstumbler beats me.
Some more info please.. OS, Driver version, If a reinstall/downgrade of the driver change the behaviour, etc etc
Just my 0.02€'s worth...
Dutch