Inline 6 no reverse...

[zmoz]

Last year, before I turned my boat into a drift boat, I was having some problems with reverse. ('74 inline 6) When I bought it last year, it had no reverse. The previous owner gave me a reciept for ~$900 worth of work on a "reverse problem". I adjusted the shift cables, and it worked great for a short time. Then, it would reverse fine for a few seconds, and then kind of jump out of gear for a second, and go back to working fine again. By the end of the year, it would make a horrible grinding noise the whole time you were in reverse, although it still went in reverse.

Any ideas what's wrong? Right now the lower unit is detached, I'd like to get this problem fixed before my new powerhead gets herre if I can.