I have the exact same issue, though already reported months ago here: goo.gl/U99kpe
It has nothing to do with bios 1.4.1 and 1.5.0 and mostly preexisted way before that. I was always wondering, why in some games it didnt boost up and kept at 99% GPU load, good low temps, but bad FPS. Then randomly it started to work and then randomly fall back. I never got on the point, that it might be TOO LOW temperature related because they were always good around 45-48°c...
If this is true, it is ridiculous. So if dGPU temps fall below 48°C... it throttles DOWN... regardless of the load it has? This cant be true. But it would explain a lot.
Because I try a lot to optimize my XPS 15... undervolt, set all to low in games, fps cap at 60, and I dont game intensive games anyway. That could explain, that I optimized the laptop "too good", and then it throttles because of this? This is pathetic.
Oh and btw... reduce the damn reCaptcha behavior I had to solve 30 RECAPTCHES BEFORE I COULD POST. Or switch to the new reCaptcha 3.0