The "D" light is blinking intermittently?

Hello,

Typically you will get shuddering transmission due to low fluid or contaminated/dirty fluid.
First check the fluid level. Make sure it is at the correct level and in good condition.
If the fluid is low then the transmission will not shift properly due to low fluid pressure. Usually the shuddering happens in the higher gear.

When you check the fluid level, make sure that the vehicle is at operating temperature then shut off the engine. Also the vehicle need to be leveled. I have included the factory transmission fluid check procedure for your reference.

https://www.2carpros.com/articles/automatic-transmission-problems

If the fluid is dirty you will need to change the fluid.

Here is a helpful guide:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.