After driving in highway for 10 minutes or longer, the engine meter starts moving back and force, the vehicle got some "pushing and dragging" effect, feel like it is constantly changing gears. after that, this effect come and go, the engine meter stay at 0 most of the time, or jump irregularly. Originally i thought it's the transmission problem, but lately i noticed that i have to let the engine cool down, and than re-start, it will run another 10 minutes or so before seeing this problem.
Is it engine collant gone ? or some thermo sensor mulfunctioning? or spark plug? distributor cap / wireing? Anybody can give me some hints before i take it to the mechanics ?

Thanks!