1990 Dodge Grand Caravan - Goes into limp mode

the transmission is fine until down shifting, then it goes into limp home mode, then I stop the car, restart engine then transmission is ok, what is causing this problem ?

When you say “downshifting” do you mean you’re manually downshifting or just the transmission downshifts on its own? Either way, take it to a good independent transmission shop but bear in mind that you’re talking about a 30 year old minivan. I wouldn’t put much money into it.