* Solenoid Problems: Transmission solenoids control fluid flow within the transmission. A sticking or intermittently failing solenoid (shift solenoids are most likely) can cause rough shifts. The heat of operation may cause them to stick, then cool down enough during the restart to work correctly for a while.
* Transmission Fluid Temperature: Thick, cold transmission fluid can contribute to sluggish and harsh shifting. If the fluid isn't warming up correctly, or there's a problem with the fluid's condition (contamination, low level), this could be the cause of the intermittent roughness. It might be particularly noticeable when the car is cold. Once the fluid warms up to its operating temperature, the shifting improves.
* Electrical Connection Issue: A loose or corroded connection in the transmission's electrical wiring harness can cause intermittent problems. The vibration of driving might cause a temporary break in the connection, leading to rough shifts. Restarting the car may re-establish the connection.
* Low Transmission Fluid: While less likely to be *intermittent*, low transmission fluid can cause rough shifting, and the fluid level could subtly change due to a slow leak.
* Valve Body Issues (less likely): Problems within the transmission's valve body, such as a worn valve or passage, could cause intermittent shifting problems. However, this is less likely to be completely resolved by a simple restart.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the easiest and most important first step. Check the fluid level with the engine running and warm. Note the fluid's color and smell; dark, burnt-smelling fluid indicates a problem. Low fluid needs immediate attention.
2. Scan for Codes: Use an OBD-II scanner to check for any diagnostic trouble codes (DTCs) stored in the transmission control module (TCM). These codes can pinpoint specific problems.
3. Inspect Wiring Harness: Visually inspect the transmission wiring harness for any loose, damaged, or corroded connections. Pay close attention to connectors near the transmission itself.
4. Consider a Professional Diagnosis: If the above steps don't reveal the cause, it's best to take the vehicle to a transmission specialist or a reputable mechanic. They have the tools and expertise to diagnose more complex issues, including solenoid testing or further inspection of the valve body.
Important Note: Ignoring a rough-shifting transmission can lead to more serious and costly damage. Address the issue promptly to prevent further problems.
