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Too much lump should not be an issue. I often fill to the top of the fire RING above the fire box. Are you adding to used lump? Have you stirred the old lump to clear the grate? What are your vent settings? How much ash in in the bottom? What size Egg? Sounds like an air flow issue.
If I'm going real low and slow (below 275) I use the daisy wheel, just because it is easier for me to maintain a low temp with it on. Anything 300 or above and I am topless and just use the bottom vent to control temp with no problem.