Not sure if its me, but, I think you took off while you were winding up the engine not allowing your engine to produce a good initial pull off of the line. Also the engine sounds like its seems to not be fully 'opened' or maybe more accurately, have a constant-tone engine sound in either acceleration or decel during the weight increase as your progress down the tracks happens. I have not the experience to say what it is but it sounded off to me. Lack of air? Lack of air control? Is there any communication change done that could de-rate the engine? You said things looked well, we just need more opinions, more clear than mine since I did not shed much light, just opinion, that I am sure wasn't worth much.
Did you log what you had for readings?