2002 Montana Intermittently won't accelerate past 30 MPH

2002 Pontiac Montana will not accelerate after going 30 MPH. There are no check engine lights or any other signs or problems. This usually happens after driving 50 MPH or plus for 10 or miles, it decelerates and won't go above 30 until it sits for a few min turned off. At first it was just doing this for interstate driving, now it is happening more frequently.