Peter, there is a common problem often noted in the US that could beHad the same problem three years ago : box stuck in reverse when going
from 4th to 3rd... replaced the gearbox with a secondhand one from
Mazspare. This time the garage reconditioned the box, 5th and reverse
cogs were binding apparently. Should get my baby back tomorrow!
something to look at with your gearbox. The reversing switch can cause
the gearbox in earlier models to stay stuck in reverse. I believe you
can undo the reversing switch & bolt it back up.
There is probably a note about it somewhere...
...ahh, here it is
http://miata.net/garage/stuckreverse.html
cheers
H