Candle Wax Ripples

I'm having an odd problem with my candle molds. Can anyone out there help?

I make these in a standard pillar mold, with wax release spray in them so they slide out easily. Sometimes the candle has all sorts of odd ripples in it - and sometimes it is fine!

Any ideas?

Note that the really solid lines were made by actual pour changes. I.e. I'd use up all my wax on one pour and then have to wait to melt some more wax before I finished off that pillar.

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