I found stirring in some kettle boiled water to the jam just before serving helped to thin it and make it spreadable - Loved the idea of adding port too!

I'd put it in a heavy pot and stir about 1-2 tablespoons of ruby port per 8-ounce jar into it over medium heat. It's a great combination!! The other thing you could do, if you'd vacuum sealed the jars, would be simply to save it until you had a batch -- even of another stone fruit -- that ended up too runny, and then mix the two together. Plums are so unpredictable. Some varieties of plum have quite high levels of natural pectin, so every batch is different. Earlier this summer, I made a batch of peach jam that didn't set very well -- I had used an apple stock pectin I'd made using a recipe found online, but hadn't tested its actual pectin level using the rubbing alcohol pectin content test -- which I mixed into a very stiff batch of plum jam. It was divine.

I've overcooked jam more than a few times, and plum jam seems to thicken more quickly than others. Any overcooked jam can be revived by adding water (or fruit and sugar as amanda suggests). Just heat it all the way through--I've even dumped already processed jam into a pot with water, boiled it and re-processed it. Worth salvaging.

I'd either thin it with water (warming it on the stove), or cook some chopped fresh plums with water and sugar until they're loose and sauce-like, and add this to the overcooked jam until thinned to your liking.