Keep driving it and keep your eye on the tach and O/D Off light while this happens.
Even if it doesn’t come on, I’m willing to say it may be the MLPS (manual lever position sensor) simply because it’s got such a huge failure rate on Windstars. That sensor tells the ECM what position the gear lever is in, and when it fails it sends bogus signals to the ECM which causes upshift/downshift activity.
Torque converter clutch is another possibility, as was said.