Unexplained amonia spike

Question
Okay, I'm at a loss.  I have had fresh and salt water tanks in the past, and have never had this issue.  I have a 42 gl hex, running a Penguin Emporer filtration system with an under gravel bubble wand.  Over the past year, the tank has been haveing a ph balance of 6.4.  This occurs after a water change using water with a ph of 7.0.  After the change, the amonia will spike up to 4.0 ppm.  Then the ph drops suddenly to 6.4ppm.  Needless to say, all the fish die.  Recently, I have chagned the water weekly, usually 20 gls at a time.  There are now only 6 tetra in the tank, and the same thing is occurring.  Also, feeding is only once a day with onla few flakes.  What could be causing this?  I have asked at PetsMart, Petco, and local shops, and noone can give an answer.

Answer
I'm actually surprised you have not found any answers or possible solutions with all the places you have directed your question.
First, have you ever tested the water you are adding after a water change for the ammonia level? There could be a problem there. Secondly, you can try adding ammonia chips in a nylon to your filter system to assist in filtering out the ammonia. Thirdly when you do these water changes do you add anything to the water?? If you do not you may want to add a product called aquasafe, basically what this does is neatralizes the water it is being added to and helps keep the stress lower for the fish.
Also, unless there is a water chemical problem you should only really need to change about 20-25% of the water as a routine change. Changing the higher amount could be causing the problem as well, it may be causing your water to start cycling all over again which could account for your ammonia spike as most new tanks will have this when it cycling. I hope this has provided some direction for you, if there is anything more I can help with please drop me an email. Best of luck....