You probably have some plugged-up oil passages that are not allowing enough oil volume to the oil galleries to keep the lifters filled. Did your rebuilder take all the oil gallery plugs out and brush/flush all the passaged? That's vitally important with any engine, but it's really necessary on one with hydraulic lifters.

If new cam bearings were installed, it's also possible to obstruct certain oil passages by not getting oil holes in the bearings lined up correctly. You might be looking at another engine disassembly if the lifter-filling problem persists. Try taking the pushrod cover off and spinning the oil pump with a 1/2" drill motor, and watch the lifters to see if oil is being supplied to them properly. I'm beginning to suspect an incompetent assembly of the lower end of the engine by the rebuilder.
Jerry

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