playing the sequence out of sync- need more RAM or wrong fps

Hi

I'm editing a project 60 mins now in Avid Media Composer 5.  When I play the sequence it runs well but after one minute and half i start seeing the sequence out of sync, and it goes like this gradually. If i press STOP at any point and then PLAY again it runs in sync again and after a minute or two the same thing. 

I hope is something like having not enough RAM or something like that and not something that is seriously bad.

I'll appreciate any help with this issue

Thanks

Ricardo

Here some details about the project.

Editing in an old MacPro with 6GB RAM  , 2 x 3Hz DualCore.

it was shot in S16mm at 24fps, then transfer to PAL DV (25fps) format for offline editing.

Direct Sound recorded in Nagra V at 24fps.

The project settings that i made are:

General: 

PAL ,16mm, Film Style Editing

Film &24P setting:

Edit PlayRate : 25 fps

Master Default Film Type: 16mm

AudioTransfer rate: Film Rate 100%

The CaptureSettings:

Video Resolution DV 25P 420 MXF