1996 Lumina Gas Tank Problems

I had the gas tank of our Lumina replaced twice at the local garage in the past 6 monthes. The problem with the original tank, and the first replacement was that the tank was flexing. When you turned the car
on/off you could hear loud thud noises as the gas tank flexed. After a couple monthes of this flexing, the tank cracks and breaks. The Mechanic actually told us that we had to have drove over something like a rock and damaged the tank. This is obviously not true.
I am now on tank #3 (second replacement). The problem is still occuring, but now you can hear it flex while driving. The mechanic claims he sells many of these tanks and has not seen this problem before. Im up over $1000 already, and I dont know what to do. The car should have been taken to the GM dealer in the first place, but I am wondering if anyone here knows of something that could be tried before forking over another $1000+ to get this fixed at the dealer.
We have put new caps on the gas tank filler and that has not worked.
Has anyone heard of this happening before? Any advice would be appreciated.
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What is the sound like-- when does it happen? I have 97 Lumina, and I have strange noises coming from the area of the gas tank. Could be discribed as loud clicking, or thumping. I haven't noticed the empty/ full difference, never paid attention to it.

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I bought a '97 Lumina for my daughter. When I got it the check engine light would come on after driving it for a short distance. I took it to friend of mine that has a garage and they found some kind of a relief valve for the fuel tank to be bad. He said if the fuel tank builds up too much pressure that the valve is supposed to relieve it. I had never heard of such a thing. Now I am wondering if that is causing all of your problems, it builds up with pressure and keeps flexing the fuel tank till it finally cracks.

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