Shadeaux

I have a '98 Venture that I bought just a month or so ago, and I took it on a 400 mile trip over the weekend.
I noticed that the transmssion seems to be either slipping, or just not acting properly.
I used Cruese control most of the trip, but even without the cruese on, the problem persisted.

Somewhere between 65 and 75 MPH the transmission would drop down a gear, rev up, then shift back up to the proper gear. The speed would not change, but you could certainly hear the engine rev up. I dont have a Tack to see the RPMs tho =[

Typically, this was just a 1-2 second event, it would drop, kick back up and then be fine, but once it actually dropped down, and reved for a couple extra seconds, then kicked back into gear.

No dummy lights ever came on, and I've checked the trans fluid, it shows full. A friend that works for Autozone (an ex-mechanic) said he could plug in the lil code reader and see if there are any codes, but ive not be able to get to him yet. He said could be some sort of Vaccum Sensor for the trans.

Has anyone else had this problem? Any ideas what I can do to fix this? I was really worried that I wouldnt make it home , but its did get us home.

Thanks for any advice you may have.