Awesome.. finally popped a code for you.
P1811, if you search google or here, you'll find this to be super common to the transmission. The typical issue there is a soleniod commonly referred to as one of two names. EPC - Electronic Pressure Control or PCS - Pressure Control Soleniod. It's job is to control pressure and commonly a shift kit can help with the issue as you'll find when you search. It's a bandaid that is relatively cheap and easy to install. It may help, it may fix the issue..it may not.
Here's a link of a sponsored vendor that explains it well. Harsh Shift, P1811
The accumulator is where the shift kit is installed.