This is a sign of high Video RAM (VRAM) usage. Adding 300 high-definition cars puts a massive strain on your GPU. Try lowering your texture quality in the game settings or using a "Low Traffic" gameconfig variant.
Do not exceed 320 add-ons. The 300 number is a ceiling, not a goal. Leave a 10-20 car buffer for stability. gta-5-gameconfig-300-cars
When you add a modded car, you aren't just adding a 3D model. You are adding: This is a sign of high Video RAM (VRAM) usage
[HEAP] MaxHeapSize = 1024 ; try 1536 for 300+ cars try 1536 for 300+ cars