What possibilities on jumpy height

[RogerRabit]

I have a question about the height beeing jumpy

History

At home i fly a 1.8m 6.5kg FAI Nitro bird

For my holiday i took a old 1.06m leightweight flybar electric bird 20yo construction with 1.5kg takeoff weight with me.
Did fly bad until i changed the head to 0.78 mixing ratio and made more clearance for flybar not binding.

Today we have 30mph wind with nasty higher gusts in between.

Even on my "windy" setup curve the bird is very jumpy in height. On some gusts it is jumping more than a meter, even i am not slow on collective.
Even increasing rpm by 200 did only help a bit because the collective "grips" better

There is no horizontal jumps, neither into nor with the wind, but i have the canopy off at the moment.

My 800 sits in such a wind with a lot less corrections.

Is it just the lower disk loading? Is it too much wind for a 500?

What can i do? Add lead to increase diskload? Reduce flybar ratio? Increase? (pain to do, the head is a ..... Construction ), add flybar weights?