If it were mine, I would have an Indy do a transmission service, (look in Yelp and read the reviews) particularly if it had not been done at all or for many miles. Involves removing the trans pan, 4-5 quarts/litres of trans fluid, uses zf lifeguard 6, expensive stuff, changing the filter, cleaning the pan which has magnets that catch the fine metallic particles. These particles end at the bottom of the pan and can cause the solenoids to foul up as they are close to the bottom of the transmission. The solenoids activate the shift points. Doing the service is a good place to start. I am thinking it will be $400+ but perhaps solve your issues. This 6HP26 ZF transmission must use the ZF fluid mentioned. on the passenger side of the transmission is the mechatronic sleeve, an electronic connection to the trans...this often weeps or leaks fluid, which might also be your problem, low fluid, so doing the service is wise.