Is your check engine light on?
You could have a problem with air, fuel or spark, but any of those would probably trigger the check engine light. You could also have a problem with the transmission. I'd investigate changing all the fluid. Depending on the type of transmission, this might involve draining and filling it several times, running it in gear for a few minutes between changes. Be sure to use the official recommended Ford fluid, not generic.
But, changing the fluid will not rescue a dying transmission. It could help if the problem is worn out fluid, and it could stop whatever problem you have from getting worse. Did you change the fluid because you suspected a problem?
Ultimately you may need to take it to a shop that specializes in transmissions for a repair or rebuild, or live with it as long as you can.