Car speed resistance

Just started season 2 in my team career. Yesterday was the 1st race. Start practice 1, as im driving the top speed the car gets slower to reach top speed and not long after the cars speed will slowly increase. I was on a straight just shifted into 6th gear and it would go any faster and the all of a sudden the speed was dropping like I had a foot slightly on the brake pedal. Was not touching the brake pedal and flat on the accelerator pedal. If I change up or down gears, took my foot of the accelerator, tapped the brake pedal it would fix. It happened a number of times for me to check what would fit it. Im am using a logitech g923. I turned on assetto corsa competizione, didn't happened on that.