New horse rearing and bucking on lunge. Please help.

My one is usually perfectly behaved on the lunge but he has days were he's excited (usually because I tend to lunge rather than ride on windy days!). So he'll buck and mess around, I dont really care as long as the work gets done. He behaves perfectly under saddle (never bucked etc.). A friends horse that I ride is similar - she's been a right brat on the lunge sometimes (vertical rearing, bucking, galloping off etc.) yet she's lovely under saddle.
 
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