You need to be absolutely sure that the measurements your making is correct. Test kits can be wrong or you maybe be doing the tests wrong, they're not always straight forward. You should cross check your values with different brand of test kits or test someone else's water, preferable someone who's got stable parameters.
For all you know, your mag could be 1400 but your testing 1000. or your alk could be 15+ but your testing 10. I don't think a jump in mag between 1000 to 1300 would harm anything, so something is not right. But also, stable levels of alk should be 8-9 max, higher you go, the higher the risk. HTH, good luck.