Change in garageband on MacBookPro?

hi,
I have just upgraded from a powerbook g4 to a mac book pro, i used to be able to play and record my guitar using the line in without any problems.
However with the macbookpro there is now a lag that builds up after a few seconds. I cant understand this as the macbookpro has much better specifications, the only thing i can think of is that its the new garageband software??
This lag only appears for the amp simulation setting and makes playing and recording impossible. The buffer in preferences is set to best live performance and even when i have only GB open and only one track, there is still a delay of a second or two. is there any way around this, or should i try and get back my powerbook?? please help!!

Man, I have the exact same problem. I stopped recording music because I just can't anymore. Garageband is almost useless because of this. I've been trying to solve it for almost a year now but I never came up with anything.