just cuts out at 50?

Hello,

It sounds like the vehicle is going into limp mode (low power mode). That's why you cannot go past 35.

https://www.2carpros.com/articles/reduced-power-limp-mode


Usually when the engine is in limp mode the check engine light gets illuminated. Is your check engine light on?
If so retrieving the code(s) can help with the diagnostics.

https://youtu.be/vMwTzURWXmk

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

One of the most common cause of low power mode is due to having a faulty throttle body. Sometimes it is due to having dirty components in the throttle body. When those sensors are dirty they are unable to monitor the air flow and control the fuel mixture, this triggers the ECM to put the engine in limp mode for safety.
Many times clearing out the throttle body components can fix this issue.

Here is a helpful article:

https://www.2carpros.com/articles/throttle-actuator-service

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.