Ciao DaisyM-
I'll be the first to tell you I'm no fruit tree expert, but what I can do is share what I've observed with my own fruit trees over the 5 years of being in this house. We have several old fruit trees, exact age unknown. We have a pear and very large mulberry in the front yard, a cherry which has been poorly pruned and maintained, 5 small-fruited round plums, 2 larger-fruited round plums, 1 Italian prune, and 2 apricots. The apricots and plums act a bit biennial-ish, producing fruit in abundance every other year. The pear produces every year but I have to be über-vigilant or the birds and squirrels get most of the fruit. The mulberry is just a monster of fruit every year, but it doesn't have a very assertive flavour, so we eat them mostly straight off the tree as we wander throughout the garden. The cherry tree has been neglected and most of the fruit-bearing branches are very high up now, so it's pretty much a bird and squirrel feeder. We don't have a ladder that gets up that high. Apricots and plums are related, so that's where I would look first, to see if they normally behave that way, producing fruit every other year.