Another piece of the puzzle: it IS hitting the current limit and throttling. Under Intel XTU, with full load, it shows Current Limit Throttling at 100%. With no load, it shows 0%. I think Current Limit Throttling occurs when the motherboard is sending current above specifications. Dell has replaced the motherboard (no effect) and sent a second, identical computer (no different). I've let them know about the current limit throttling, and that they need to either send a motherboard that doesn't cause throttling, or refund my purchase. Dell's support has been super, but I'm afraid my question won't reach someone with expertise to address this issue.