Models failing under mongrel

I have a simple model (Payment Gateway) which simply represents the
attributes of a number of payment gateways.

A shopping cart controller executes a find to determine which payment
gateways are available, this is simple:

@payment_gateways = PaymentGateway.find(:all, conditions => [ 'enabled
= ?', true] )

This executes fine on all our test servers including Debian, Ubuntu and
our Macs running on webbrick, lighty and fastcgi

However, on Redhat EL4, we're running under mongrel and also trying
under mongrel cluster.

The class and method work fine after a server restart, however, as soon
as mongrel is restarted, we lose the ability to access the class and
method. We can't get the model back until we reboot the server.

Here's the error we get:

undefined method `find' for Public::PaymentGateway:Module

has anyone had any issues with these disappearing models?

Thanks in advance,