Transmission Not Go Into Gear problems of the 2003 Subaru Impreza Wrx

I own a 2003 Subaru wrx sedan with a manual transmission and quite often when shifting gears 2nd,3rd,4th and especialy in 5th gear it tends to not shift in gear(gets stuck) and grinds from time to time. This scares me when I get on the expressway and I have to shift gears to get in the flow of traffic I'm affraid that its going to stick and cause a accident (it has gotten stuck in traffic and have almost had a accident). It happens during normal driveing conditions even when I shift fron neutral to reverse it will grind. It has been a problem since I bought the car in 6-27-03 I've taken the car to the dealership and asked for on of their technician drove my car and said that there is nothing wrong when shifting gears. The car has no modifications.