2014 Hyundai Accent - Transmission and speedo off

My car some time working normal and some time giving problem in Automatic Gear transmission and also speedometer not showing any value, while Reverse gear working normal and when go to D (Drive) vehicle run but not changing the gear…However checked by Gear Mechanic and he told that no any problem in gear because car is running normal in Manual gear (1,2,3), after that checked by computer , in which no any fault showed. Even on car display no any fault indication…further sometime when I turn the car on left , the fuel gauge become down , and after that come normal…so please help me , due to that i am in tension and unable to solve this issue…If there is no gear problem, no computer problem???what I will do