Power button flashing

While driving the car the power light was flashing. When I start car it's fine. I have foot on brake and turn key then put into Drive - when I do that it makes a engaged clunk sound( normal ) ?. Well when driving and slowing down the car revved high and clunked down a gear while slowing down around corner. This wasn't normal for the car while driving. I hope I explained this ok.

Thank you

Melanie