I never had any problems with amavis on a system yet and its very widely used, but all the systes that we manage have at least 2GB RAM. Replacing amavis should not be nescessary.
How much memory does your server has?
Do you use any custom spamassassin rulesets?
You can also limit the amavis memory usage by defining the number of amavis threads running and the max number of emails feeded to amavis by postfix.