I once had a Plantronics headset that was wretched. If very much of my body was between it and the phone it would cut out. Sold that hunk of junk.

Maybe it's just your body position at speed?

Now I have an LG headset, I can leave the phone on my dresser and walk all over the house, I have to get to the far side of the basement or outside before it starts to break up.