Solution 1:
Turn off variable refresh rate (Gsync etc) for the trainer in your graphic card driver settings or in Windows.
Solution 2:
It's likely that something else has attached itself to the trainer that is using the GPU.
Solution 3:
Disable the in-game trainer overlay in the trainer settings.
User suggested Solution 1:
Open Nvidia control panel -> Manage 3D settings -> Program Settings tab
Add your trainer as a program to customize -> Preferred graphics processor, choose High-performance Nvidia processor -> Scroll down to Monitor Technology and choose Fixed Refresh -> Scroll to Preferred Refresh rate and choose Highest Available