Just one thought - is this mainly at the same location? What's your signal like there?
I've found that if my phone is showing 2-3 bars and I pick it up and hold the bottom the signal can drop to nothing. I think the antenna is in the bottom left-hand corner (from experimenting). As you pick the phone up to answer, it may be enough to lose the signal if it's weak to start of with.
Unfortunately, I have found that the Desire, although smart is not as good at being a phone as my old Nokia, I have terrible signal at home but the Nokia shows full signal all the time with the same SIM.