* Transmission Issues: At higher speeds, the transmission shifts into higher gears. Problems with the transmission itself, such as worn clutches, a slipping torque converter, or a malfunctioning valve body, could cause jerking or hesitation under load. This is a prime suspect given the speed-specific nature of the issue.
* Fuel Delivery Problems: At higher speeds and under load, the engine demands more fuel. Issues like a clogged fuel filter, a weak fuel pump (not delivering enough pressure at higher demand), or a failing fuel pressure regulator could lead to fuel starvation, causing the engine to stumble or jerk.
* Ignition System Problems: Similar to fuel delivery, a weak ignition system might struggle to provide consistent sparks at higher engine speeds and loads. This could be due to worn spark plug wires, failing ignition coil, or a distributor cap and rotor in poor condition (common on older vehicles).
* Airflow Restrictions: A partially clogged air filter can restrict airflow, especially noticeable under load at higher speeds. Also, check for any vacuum leaks in the intake system. These leaks can cause erratic engine performance.
* Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS): These sensors provide crucial information to the engine's computer about airflow and throttle position. A faulty sensor providing inaccurate readings can lead to poor engine performance, especially at higher speeds where the engine's demands change significantly.
* Oxygen Sensor (O2 Sensor): A faulty O2 sensor can affect the air/fuel mixture, causing hesitation or jerking.
* Computer/ECM Issues: While less likely, a problem with the Engine Control Module (ECM) or its connections could also cause intermittent issues that manifest at certain speeds or loads.
Troubleshooting Steps (in order of likelihood and ease):
1. Check the transmission fluid: Low, dirty, or burned transmission fluid is a common cause of transmission problems.
2. Check the fuel filter: Replace it if it's dirty or old.
3. Inspect and test the spark plug wires, distributor cap, and rotor (if applicable): Look for cracks or corrosion.
4. Check the air filter: Replace if dirty.
5. Have the fuel pressure checked: This requires a fuel pressure gauge and is best done by a mechanic.
6. Inspect for vacuum leaks: Listen carefully around the intake manifold and vacuum lines for hissing sounds.
7. Have a mechanic test the MAF sensor, TPS, and O2 sensor: These require specialized tools and testing procedures.
It's highly recommended to have a qualified mechanic diagnose the problem. They have the tools and experience to pinpoint the exact cause efficiently and safely. Trying to troubleshoot these issues without the proper knowledge and tools can be dangerous and potentially lead to further damage. The fact that it happens at a specific speed makes it less likely to be a simple fix like a loose wire.
