It is very difficult to find the 3rd and 5th gears and now the engine is hesitating a lot in the higher gears like the gas line is blocked or it is running only on 3 cylinders. Had my mechanic look at it and he could not find anything wrong. Also the check engine light was not coming on so l am really wonderingif it is the transmission that is going.

The clutch could be slipping. In 5th gear going 80mph floor the throttle and see if the RPM goes up without the car moving forward.

Also, check to see if the linkage needs adjusting

did u check ur gearbox oil level? my subaru did that when the turbo boost controler went out. is yours an impreza wrx? because my car did the same thing l have an 02 wrx impreza & the boost controler went out & when l got into higher gears my turbo wasnt putting out any pressure so in the higher gears it lagged bad. l had no power. the boost controler is on the pasenger side top left corner of the engine compartment, its underneath a little black cover next to the strut tower. the new boost controller will cost about 150 bucks!