* Transmission Fluid Heat: Highway driving generates significant heat in the transmission fluid. If the fluid is old, low, or contaminated, it can lose its lubricating properties when hot, leading to harsh shifts. The cool-down period after stopping allows the fluid to cool and regain some effectiveness.
* Low Transmission Fluid Level: Low fluid level exacerbates the heat problem. The transmission may be unable to properly lubricate and maintain pressure when the fluid is low, especially under heat stress. Check your transmission fluid level immediately – this is the first thing to check. Make sure the engine is running and the car is level.
* Transmission Fluid Contamination: Contaminated fluid (from wear and tear, or a previous leak) can also lose its lubricating properties and contribute to harsh shifting. This is more likely a gradual problem, not a sudden one.
* Solenoid Problems (Less Likely): While less likely to be the sole cause of your specific issue, worn or failing solenoids within the transmission could contribute. Solenoids control fluid flow within the transmission, and malfunctioning ones could cause erratic shifting. Heat could exacerbate the problem, but a complete failure usually results in more severe symptoms.
* Valve Body Issues (Less Likely): A problem within the transmission's valve body (which controls fluid pressure) can also cause shifting issues. However, this is generally a more persistent problem, not one that improves after a short stop.
What to do:
1. Check the Transmission Fluid: This is the most important step. Check the fluid level and condition. Is it low? Is it dark brown or burnt smelling (indicating it's old and needs changing)? If it's low, immediately add the correct type of transmission fluid (consult your owner's manual). If it's dark and burnt, it needs to be flushed and replaced.
2. Inspect for Leaks: Look carefully around the transmission for any signs of leaks.
3. Consider a Transmission Fluid Change/Flush: Even if the fluid level seems okay, a complete fluid change and filter replacement (if applicable) are highly recommended given your symptoms. Old fluid is a primary suspect.
4. Professional Diagnosis: If the problem persists after checking the fluid, you need a professional mechanic to diagnose the issue. They can inspect the transmission for more serious internal problems.
Ignoring the problem could lead to more significant and costly transmission damage. Addressing the fluid level and condition should be your immediate priority.
