The best GPS signal reception can be reached when training in an open area where there are no obstructions for the satellite signals. Due to the nature of the GPS signal, for example hills, tall buildings, and trees may block the satellite signal. Rain, fog, and snow may also affect the signal quality. These conditions may result in uneven speed reading during the exercise and cause an uneven speed curve in Polar software even when you have exercised going at a steady pace.
If you are training a lot in an environment where the signal can get blocked, a reliable way of getting a speed reading is to set automatic lap to 500m or 1km and check the average speed for the lap when your wrist unit takes the lap and beeps.
If you are exercising on a forest road, try changing the sensor to your roadside arm to see if it helps with the signal reception.