My GMC V8 did the same thing. I have read that the oil takes time to get to the valves, etc. Some people believe it is the oil filter letting oil drain when it shouldnt. Try a nicer oil filter, maybe you rebuilder used a cheap one?
who knows, it never stopped in my GMC but never caused any problems


Sold Truck: 89 DLX extra cab 4x4 22re Automatic/ New truck 97 T100 4x4 auto Sr5