Axehigh, have you tried curing the mould in an oven?? it seemed to work for the gu doing the train.
About the plaster: i think you might wanna try more powder.
Alternativly add some PVA into the mix. Might work, i never tried it though.
I know that plaster bandages can 'age', might be the same with powder. I guess you're living near the coast, so humidity can be a problem.Better luck with your next castings!!