The primary problem of miniature air shocks is that they rely so heavily on the seal, which is heavily affected by coefficients of expansion and the seal itself. Many shocks listed as air dampers are more correctly friction dampers of the rubber o ring generating heat.
Could you please state which aluminium alloy they are made of, whether they are anodized/hard anodized and teflon coated and the piston and seal material, as if these haven't been chosen with coefficients of expansion in mind, the performance will change under heat and usage.
Air shocks are much more susceptible to any dirt inside causing seal damage. Please detail the o-ring seal you use as any wear changes characteristics, and what proportion of damping is seal friction heating in your tests.