Control Box shifting cable

[Ryanshepherd1]

So I had my lower shift cable replaced and have had it back to the mechanic twice for shifting issues. The last time he told me that my shift cable had some slack going from the control box to the engine but he adjusted it as best as possible but it was maxed out. I just took it to the lake and it went right into reverse no problem. Drove it around for a bit and then it wouldn't go into reverse (I could hear the gears clicking). so I drove it around more and went right into reverse again and on and on.... I called the mechanic today and questioned him on if he had the Jig and proper tools for an OMC, he said yes. he is telling me the issue has to be the shift cable from the control box back to the engine. I think I can do this myself but want some Opinions, do we believe him? Please help. This boat will Kill Me.....
Also how to identify what cable. 1989 2150 Bayliner with OMC