Brains suggestion is definately the way to go initially.
Create a new file ini-custom.inc.php in the root of your store.
I did have issues with the memory size when i did this, so you might want to play with this.
<?php
# Custom initialization commands
# Filename: ini-custom.inc.php
# File location: main CubeCart folder
ini_set('memory_limit', '256M');
ini_set('max_execution_time', '60');
ini_set('error_log', 'error_log');
ini_set('log_errors', 'on');
?>
To add to this, Cubecart support is extermely fair.
You'd be hard pushed to find dedicated product support at such a price anywhere else.
https://www.cubecart.com/technical-support