99 Windstar tran slips when accelerating

Tran recently started slipping(?) when coming up to speed. Happens most often when resuming speed with speed control. It starts to accelerate, then hesitates, then "thunks" into gear and runs OK. Also will "thunk" when
getting up to speed from getting up to speed from a stop. Car has 85 thousand on it and has had the trans serviced two years ago. Usually runs good. Is this a simple fix or should I trade it? Any help appreciated because I haven't a clue. Thanks.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

then
My 95 (230k kms) has the same problem, and has had it for about 4 years. It's annoying and alarming at times but doesn't seem to bother anything much because it hasn't gotten any worse. It is worse in the colder weather and the slipping continues until the tranny gets up to temp. I was told it was "perhaps" the valve body, but $800 is too much to pay for an "educated guess". Personally, I think it is an electronic speed sensor or pressure transducer at fault.
Let me know what you find out!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.