Bad texture quality on cars

[TMCharly]

Hello,
about 2 months ago a bug appeared on my game and I have absolutely no idea how to fix it.

If a texture of a car it bigger than 1024px² the problem appears.

So with a default skin (1024px²) no problem.
With a 2048px² texture it appears randomly (when the texture is downsized it never go back to its normal quality).
And with a 4096x2048px texture or more its never good.

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My specs:
GPU: AMD Radeon HD5450
CPU: AMD Athlon Dual Core 4450e x64 2.3Ghz
3Gb RAM
Windows 7 64bits