Attitude and customer service are both "top down" issues.
Next time you get crappy service, be patient, get your part and pay for it, then ask to speak to the manager. Be polite, no matter how mad you are. State the facts that just happened, not something from six weeks back, and be ready for the big question--"What do you want me to do about it?". Firing the offending employee probably isn't going to happen, at least not right there in front of you. Just ask the manager to handle it in accordance with NAPA employee policies, and let the manager know that you will make a follow up call to NAPA headquarters. And make that call. Note the date, time, the offending employee(s) name and the name of the manager you talked to. You will get action, and the store will improve. Just don't expect instant results. And have the cajones to keep going back to that store.