Water Chemistry

I am having a real problem with my alk levels. I use API test kits. When testing for alk (kh) I can do 3 test in a row and get 3 differnent readings. Just had a test done at a lfs and they gave me a reading of 13 alk. Pretty high, which would explain some burnt tips. I just tested again with the api kit and got a reading of 8. Something is amiss obviously. What test kit do you guys use and feel is the most dependable? This is a 99% sps tank.