What parts I need to change in avtomatic transmition?

Scan the computer for code/s if none bring it in and have it checked by a transmission specialist

The most common cause for such problems are the valve body. It could be the solenoids are working inconsistently or the valve body requires rebuilding.

Inconsistent solenoid does not necessarily trigger trouble codes. The voltage are correct, just that the solenoids might be slow to response.

Some components tends to stick after parking and would free sup after some running. The fluid change made some differences, and that is one of the indication of the fault with the valve body, on which the control solenoids are located.

What scan tool did you use for checking the transmission trouble codes? I usually have problem using my scan tool to check for trouble codes on Mitsubishi vehicles and only manual retrieval would produce codes.

Solenoids can work sluggish during cold fluid temperature and perform better when it warms up so it is not surprising to have the transmissuion working well only after warming up.

Abnormal cold trans fluid temperature erratic operations can also be due to fluid line pressure loss cause by worn/aged clutch sealing rings.

Incorrect speed sensor readings can cause harsh or erratic readings but at speedometers might not show accurate readings at very low speeds. Is your vehicle equipped with cable driven or electrical controlled vehicle speed sensor?

One thing you can try, check engine to body ground circuit connections, ensure they are clean and tight. When ground circuit connections deteriorates in capacity, they can cause abnormalities with engine and/or erratic transmission operations.

Harsh shifting and slipping are mostly due to faults with the valve body/solenoids but you cannot rule out pressure leakages.

If the harsh shifting has a temporarily pause as if there is temporary loss of contact before gears engaging, it could be due to pressure loss, meaning a fault with the trans and requires a rebuild.