The servo is locking up or your reverse linkage is locking up?
The servo is a very fragile servo. It's plastic geared and from the factory, the linkage is setup in such a way that it's always under pressure. I adjusted my linkage so when in forward gear, there was just enough pressure to put it in gear and hold it in gear. When I'd switch to reverse, it was under quite a bit of pressure, but I was never in reverse that long.
Another thing to do is remove reverse and get a forward only kit. Then you won't have to worry about the reverse servo as it gets removed.