Ef 300 is ism

It is more than probable that you have an issue with the lens. I would try the AF microajustment and hopefully you'll solve your problem on your new camera. The setting is "in camera" so you'll have to reproduce the setting if you change camera. Luckily all new cameras have that possibly now on the semi-pro and pro bodies.
Do not despair, it can be solved

Cheers

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Phil (Nice, FR)
www.pbase.com/webman06
www.eosdslr.com