What is this problem?

so i have had my camera for about two years now and out the box it ws fine then over time my play back will start to become grainy i have proper lighting and a high speed card and my iso are low .. i pretty much did everything with the camera even called canon themselves and they told me jsut to buy a new one which i feel they just want my money .. but i myself feel it could be a diff problem such as a sensor problem or something else can anyone help me ?