If the Jam or Syrup is simply crystalizing, then you simply have a too high sugar:water ratio; the sugar is dissolved at 100 C but oversaturates and so crashes out of solution when it cools to room temperature. If this is the case, then heat your jar in boiling water to soften it, then pour it into a pan and reboil it, adding a little extra water. Then when it cools it will simply be a thick syrup, and shouldn't crystalise (Think how honey crystalises). Careful, if you cool it in your fridge it may still crystalize.
If your jam is using gelatine or pectin then this may not work at all, though you should be able to get a syrup at the very least.
Tip: If you know the amount of sugar in your Jam and the weight of fruit, by measuring the weight of your solution you can test at 100 C if it will oversaturate: The maximum solubility of sugar in water at Room Temperature is 2 grams per mililitre of water. 1 ml of water = 1 gram so your solution should weigh just over 1.5 times the mass of sugar you used. Aim to be below this but as close as possible to it (maybe 1.8).