John, I've had two Jaydees and they were like night and day. The first was a Series 1 Supernatural that someone (not John ) had shaved the neck on and I had lots of problems with it. The second (a later MK) was a fabulous guitar, one of the best I've ever owned. Unfortunately I had to let it go soon after buying because of the weight, but it both played and sounded superb. Re the fingerboard problems, I've had exactly the same thing with Wals. And if you speak to Roger TVR about his Alembic, even the best can have problems...
The guy who my first JD was built for said that John was sold a bad batch of wood (sometime in the 80s I believe) that caused some of the problems you note. Also when Mark King took off John had to get in more help in order to cope with the vast amount of orders, and I think even John admits that the quality wasn't always there at that time; he said it was a situation he hated and he is now much happier having a two man operation.
The best-playing bass I've ever played was a newer Jaydee that was in the Gallery in Camden a few years back; it played beyond anything else I've ever tried, and sounded superb ( boy was it heavy though!). I've played many, many Jaydees over the years and I'd say 50% of them I haven't liked, and 50% have been great. However I would also say the same about the 70s Alembic Series basses I've played. I've played 2 I really liked, 1 of which was one of the best instruments I've ever played, and another 2 that (with all due respect!) I absolutely hated. One, in a shop in Haight-Ashbury that I forget the name of, was one of the most unpleasant playing experiences I've ever had, and yet I would never dream of painting all other Alembics with that brush, as I've played so many that are just unbelievable.
The same goes for every other make; I've played horrible, horrible Wals and very nice ones. I've played many wonderful Rickenbackers and just as many dogs. Some bad experiences do not a bad brand make. Some people complain about the necks on Rickenbackers but I've never had a problem with a Ric neck in 28 years of playing, and I actually find them far more stable than any other basses, my Alembic included. I used to have more problems with my Statuses!
Fandabge; Mark King had nothing to do with the look of the Jaydee as we know it. Mark's first Jaydee was bought by him in a shop in London; it was on consignment there. It had been built by John for Stanley Clarke; but apparently Stanley made it clear he didn't expect to pay for it (that's the story anyway). So John completed it and put it in the shop for sale.
Mark had just received an advance and was bass-hunting; he saw the bass in the shop and thought it looked a bit like an Alembic (in his own words, his wildest dream at the time) and made an offer; the shop phoned John who accepted the offer. Mark later had input into certain technical aspects of the basses, but the look was already there before he came along (see also the posts above re the first Jaydee bass).