Transmission? Drives forward a few feet and then it jumps back?

Good day, a problem arose when I was sitting in traffic today, my vehicle listed above 1600 made a loud noise when I was in first gear, it felt like the brakes had seized up and wouldn't move, but when I started it again, I noticed it drives forward a few feet and then it jumps back like there are bricks in front of my wheels. This happened even if I tried any other gears including reverse. So, I took it out of gear and tried to push it off the road but wouldn't move forward or backwards. I called a mechanic and he said to push it backwards whilst changing gears, heard a pop sound and it drove fine again, but every few km/miles it does it again. What is the problem? Anyone? The mechanic is out of town and doesn't really know what the problem is.