More salt. If you are tempted to cut out salt because of modern thinking, try increasing it again, just for a while.
If you are awake as it starts, it is possible to really concentrate on relaxing it, and sometimes achieve an immediate unlocking. I had a bad spell while doing sports when i was around 30, and i found that i could just get control of it about 70% of the time. I imagined the muscles relaxing, blood flowing easily through the leg, in other word mental imagery. The concentration had to be absolute.
There was not the worry about salt then and i poured it on everything. The problem stopped almost right away.