I-movie space struggle

I have never made a project in i-movie HD exceeding 20 minutes - which is still long if the project involves many transitions, effects and sound. The project I am working on is for my first customer and I need to finish it so I can get paid as money is running low.

The project is just over half way but my macbook pro is struggling after 27 minutes, and during playback the sound is now suffering.

I have deleted many files and unused applications to make more space but it doesn't seem to make much space in i-movie HD.

I have tried exportig the project and working from a hard drive but once on a hard drive it for some reason can not render fast and slow mo clips.

I now have a copy on hard drive and the sound on both copy and master is choppy and distorted. I realise that when burned to i-DVD it will probably be fine but I need to finish it first and I have at least another 20 minutes of material to add before this is complete.

I realise that ideally I should be working in FCP and from an i-mac not a laptop but I can't afford an i-mac at this stage,the laptop is all I have and I exported FCP to hard drive to make more room (which I have been told has probably spoiled it but I never had the time to learn it anyway).

What is causing my problems and how can I finish this project with the same ease in which I started it?