How do I replace the transmission range sensor in a ford windstar 2000

Set the parking brake and shift the trans selector to neutral.
Remove the shift cable from the shift arm with a suitable tool. It should pop up and off.
Remove the shift arm from the transmission shift shaft. Avoid moving the shift shaft during this stage.
Remove the switch from the trans housing.

Attachment 6117

Reinstall all the items previously removed.
Be prepared to adjust the switch so the vehicle only start in park or neutral.
There is a tool for presetting this adjustment but if you're careful and "lay" the switch where the faulty one was, this will reduce guessing.
EXCERSIZE CAUTION! KEEP THE PARK BRAKE FIRMLY SET UNTIL ALL ADJUSTMENTS ARE COMPLETE!

Sometimes I have to put the shift lever into 2nd to get the normal drive gear, and other times everything is normal by just putting the shift lever into drive.
What is wrong?

Yestome, it could be a bad TRS or one that is out of adjustment. The shift linkage could also be out of adjustment--too much slop.