Change the way product availability is checked on modified oscommerce b2b shop

There's an issue on a website that uses a webservice to check the availability of products. When opening the product pages, it checks the actual status of the product. That works. So the webservice itself is working. In the product list however, the availability is (should be) pulled from the data in the products table (for load balancing purpose). This works great when products have just been exported from the ERP. Problem seems to be when the last export from the ERP is more than 1 day old. Then all products in the listing are "unavailable". When opening them, the webservice corrects that, but that's too late.