Garageband 11 Latancy

Hi
When playing my guitar through my iMac with Garageband 11 I get increasing amounts of latency.
I am only using one track and have shut down everything else. I have tried connecting with a 6.3mm to 3.5 adapter and a cheap USB interface,. both have the same problem.
Is there anything I can do or do I need a more expensive USB interface.
If so, can anyone recommend an interface that is not to expensive as its only for occasional use
Thanks

Mid 2011 iMac
12GB ram
OS X 10.8.2

Help anybody

Try changing the input source to something else and back again. I've heard of this before and sometimes that fixes it.

Go to Garageband -> preferences

change the input source to something else then back to your interface.

See if that fixes it.

If that doesn't work then see if your interface manufacturer has any available new driver/software updates on their website.

If not, you might need a new interface.

How much RAM do you have in this machine?

How much free hard drive space on the boot drive?

Thanks Adric changing the import source works for a while but eventually it gets so bad I have to shut down the machine.
Chas_m I have 12GB of ram and 818GB free on my boot drive so don't think this is the answer.
I think it must be the crappy interface I have.
Thanks for the reply's both