Readings always start out and circle around the correct value, but as time goes by (1 min...2 mins...3 or more) the readings keep increasing. For example they start around 56.1 but end up going up to 58 or even 59.3.
Which readings? The temperature? The humidity? Forget the DHT11 (+/-2°C) if you want accurate temperature readings, forget both if you need accurate humidity readings (+/-5%RH).