10gmc

Looking for suggestions to repair an engine misfire on #1 and occasionally #4 cylinder. Especially noticeable at low idle. This condition started at approximately 40,000 miles and after 3-4 attempts to repair is still reoccurring every 15,000 miles...now with 107,000 miles. Dealer has replaced valve covers twice, camshaft and lifters and last top engine cleaner and another new plug. The dealer has been great and seems as frustrated as I am. I have read many other forums with suggestions but still haven't found an answer. Amy help would be appreciated!!!