I heat vegetable oil in a fairly narrow, high light-weighted saucepan. It only takes minutes to bring oil to deep frying temperature (180 C / 360 F) and the first batch of meat turns out crispy. But after removing the meat, I keep the heat on full strength but the oil just doesn't return to the same high temperature, staying at around 130 C (260 F), even after a long wait. Why does this happen? I find it very weird that it doesn't return to high temperature.