Recording Audio On Imac????? Options

[Jackal]

Hi, I've been playing guitar for a few years now and want to record some simple tracks on my iMac. I always thought this was supposed to be easy but am now having all the trouble in the world, I'm sure it is something small and obvious so I hope someone can help me.

I have an iMac 400 DV with a G3 processor, 320Mb of RAM and plenty of hard drive space left. I plug my guitar into my amp, run a lead from the audio-out on my amp to the audio in on the iMac.

The audio settings have been set to use the Audio In...prettly simple stuff...I thought.

I have a copy of Cubase on the computer and whenever I try to simply record a simple track through Cubase it will record a very patchy audio file, IE: you hear 2 or 3 seconds of audio followed by a total blank period for several seconds, another couple of seconds of audio followed by blank and so on.....

What am I doing wrong??????

Do I need to upgrade the Audio Card or is it maybe the processor simply can't handle recording something through Cubase? Sure, this is no longer a great computer I am using but I'm not asking it to split the atom, just record a couple of tracks.

I'm not too keen to spend a lot of money only to find out that the problem was caused by something else such as a simple setting in Cubase or something so any ideas would be greatly appreciated.

Thanks in advance...