My ED seat did the same thing. The cog (approx 40-50mm) in the height locking mechanism wears down, causing the locking mechanism to slip. You could do the el-cheapo thing and move the cogs position (rotate it, after removal) so that the teeth mesh and it locks in place. Or replace it. To get to this cog you have to remove the plastic cover, remove the 10mm bolt, and prise the mechanism off it's shaft (making sure you remember which position it was in). Before you replace the mechanism make sure its lubricated, as mine was a bit sticky when i replaced it. After your finished, your seat should lock into position.
Hope this was of some help.
BOSS 5.4lt Quad cam V8 is mine
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