My 2004 Taurus with 49k miles is having difficulty shifting gears. I have to increase the RPMs to about 3k and then take my foot off the accelerator for it to change to higher gears. I took it to a reliable transmission shop, and the experienced transmission mechanic told me to take it somewhere to diagnose what he believes is a low-power issue. Any advice on how to proceed to check for this issue? P1000 consistently shows up.