My vans not feeling right!!!?

I have a 2000 ford windstar SELwith 113000 miles. The Engine runs top notch but it feel like When you take off the Transmisson slips for a second the runs fine after the take off it also does the ford shutter at 40 to 60 mph, but not all the time. It has just this week felt like when I slow to 20 or 30 to go around a curve or turn it shifts hard into 2nd. Any thoughts at all. Its Scaring me I don't want to Replace it ....:banghead:

One SUPER important thing to do is keep the fluid changed and in good shape.
It needs to be changed AT LEAST every 30K miles.
However, as with most modern vehicles, the cooling for the transmission fluid is seriously lacking.
Heat causes the fluid to break down, and then varnish deposits begin to form and other problems as well.
So....in short, I would change the ATF using a high quality Mercon V rated fluid.
A good fluid that is readily availabe is Mobil 1 ATF.