1994 Other Oldsmobile Models Losing Power

I have a 3.8 V6 Oldsmobile Silhouette that is having power loss problems. When I drive on the interstate, it will all the sudden lose power, the tachometer and speedometer shake and the speed decreases rapidly. I initially was told it was the transmission. I had it replaced. The problem is still happening. I had the computer code read and was told the ignition module was bad. I replaced it and it does crank better, but I still have the same problems on the interstate with it losing power rapidly. At speeds in town, the van runs fine. Do you have any suggestions? Thanks.