My Holden Astra's ice mode comes on randomly

This is really backwards, isn’t it? The ice warning is designed to alert the driver to road conditions cold enough for ice to form and cause a skidding risk. But in your case, the opposite is happening and the car thinks it’s freezing outside whenever the temperature creeps up to 28. This would make most mechanics think that the temperature sensor that reads the ambient temperature has gone haywire and is telling the onboard computer lies.

Either that, or the vehicle is selecting the ice and snow driving mode on its own behalf, and that’s what’s limiting your engine speed. That wouldn’t be dependent on the ambient temperature reaching 28 degrees, however. But if the ice and snow driving mode is part of the problem, you could have a faulty switch or even a problem with the car’s body computer. An auto electrician is your best bet.