99 Intrigue not running like it should

The 3.5 in my Intrigue started hesitating, so I had the plugs changed - it has 109000 miles on it, and I believe they were the originals Mechanic said they didn't look that bad. It seems to hesitate just sometimes when taking off. Was on the interstate the other day and when it got to 75 MPH or so, it started surging and hesitating non-stop, enough to make you seasick. When sitting still, if I rev it up and hold it there, it surges and hesitates. That kind of tells me it's not the transmission. The air filter and fuel filter are new, and it's been maintained properly, oil changes every 3000 miles or less. Hope someone can give me a few ideas, before I decide to drive it into the Gulf. Thanks

ckp or fpr since no OBD2 codes.