I have the same problem as above. My ECF rating is about 70, which in FIDE terms would be about 1200 to 1250, despite having an IQ of 130- something.
I think it's nerves; I almost always get an advantage, often a decisive one, against 100+ rated opponents (1400-1500 in FIDE terms) and then start to panic about making a mistake, with the inevitable result that I do do so.