When I had the OEM cables, my fore/aft cable started slipping. That plastic locking clamp that is used to adjust it was wearing out. When I shifted agressively from 1st or 3rd to 2nd or 4th, it would slip, so I would pull the shifter as far back as it could physically move, but I got no engagement in those gears. Because of the design of the cables, and their length, I still got full engagement in 1st, 3rd, and 5th. The cable didn't slip when I moved the lever forward, only when I pulled it back, and only when I pulled it back relatively hard. The problem had started when I (mistakenly) allowed my son to drive the car once or twice. He had trouble finding reverse, so he went hard aft on the lever repeatedly trying to get it in that gear. In doing so, he started it slipping, and rapidly wore down the little plastic ribs on the inside of the locking sleeve. The more he did it, the worse it got, and it got bad fast. The difference between what mine did and what you describe, is that I could still drive using all gears, as long as I was gentle shifting into 2, 4, and R. I do think, however, that you could be having a similar problem. Does the shift lever move a lot farther back than it used to when you try to go into 2, 4, R?
BTW, check out this thread. It may help.
http://lotuselancentral.com/forum/viewtopic.php?t=9483