[Sniper Steve]

Well if it does go to semi-auto some times then it may not be a broken cut-off lever but rather the lever is not able to move freely. Its hard to tell without seeing it but if the rifle engages the safety fine and fires full-auto then the selector plate and fire selector switch are fine.

About opening the mechbox. Are you mechanically inclined? Have you worked on engines or electronics or anything like that? The mech box is not really that complicated. The only tricky thing is timing the gears. But if your good you can change the cut-off lever without having to time the gears if you take it apart right. (Assuming that is the problem) If you do take it apart make sure you take note of what goes where and how it is placed.

Where are you located?

If your are able to actually acheive semi then there is most likely something wrong with the spring that connects the selector plate to the cut-off lever or the cut-off lever is having trouble moving freely.

What is the nature of the gun? Is it brand new ish? As in are you the first owner? Has it had any upgrades? Such as a steel upgrade cut-off lever? The reason I ask is that I needed to sand a bit of the inside of the hole that the cut-off lever rotates on from the replacement I bought. The reason was that the cut-off lever did not rotate properly and this would have caused problems engaging semi-auto.