caravan - sluggish acceleration after stop

94 Caravan SE, auto. 18K.

Sometimes after a stop (either red light or stop sign), I step on gas, but
the car kinda refuses to go. It moves, but gain speed up intermittently, and
I can hear clunking noise from beneath. This just lasts for a few seconds,
and then gets to normal speed and go.

Strangely, it often happens at the first stop after the car was left parking
for a few hours. For example, on my way to work each morning, it always
happens at the first red light. Then, rest of the trip is mostly ok.

Any advice. Thanks.

Simon