Thunder/fireworks

Hi, I've had my seven year old Labrador for two years now, at some time in her past she was a working gun dog. When we first got her she was absolutely fine with fireworks not sure about thunder as none around at the time. Then last new year she became very agitated and panting with the fireworks, pacing about and trying to climb on me. This Easter while away in our caravan we had a huge/loud Thunder storm which didn't bother her at the time but since then she has become very distressed with thunder. Now this November we feel that we had made head way with the fireworks, Zylkene tablets, a den, Dap spray and us acting normal and we have had a calm weekend. I (my vet also intimated) have my concerns then that it is not the loudness of the thunder but the build up or change in the air pressure, has any one had any experience of this and have they managed to over come this problem? The vet states that Meg is fit and well in all other respects and her heart should be able to cope with the stress but its just not nice to see her so upset.