2004 Mountaineer hesitates when downshifting going up a hill and looses power, is this a transmission issue? It has 99,200 miles on it and is in great shape.

Well pczapp has covered about every single thing that could go wrong with your vehicle by the time you replace every thing that he said it would be cheaper to bye another vehicle.regards,agwbk

What is in great shape? You may have multipule issues

At 99,000 miles your O2 Sensors are worn out
Your TPS Sensor could be effecting shift stratagy
Your MAF Sensor may need cleaning
Spark plugs are only good until 60,000 miles
Transmission Solenoid Contol Packs,go bad on fords
Egr Valves don't last long on a ford
If you don't change trans fluid on all vehicles every 3 years
or 30,000 miles your in trouble,power steering fluid also,
or loose a steering rack
Is the gas filter old and partially clogged?
You may have an ignition coil going bad,very,very coming
on fords and you get one a year or more

You just need to do preventative maintenance and diagnostic
testing once a year,rather than driving several years until you
have issues then get upset.
Todays vehicles need constant supervision