If the ABS filament is aged, which certainly is likely due to the overall age of the product in general, you may be partially suffering from moisture. Consider to dehydrate the filament. I use a food dehydrator from which I have cut away the internal shelf plastic. At the highest temperature, about 15 hours suffices to dry the filament without melting it. There are entertaining photos of rolls of filament in ovens that resemble works of abstract art!