grandprix747

Hi I have a 2001 grandprix gtp supercharged, I have two problems. My check engine light will come on and flash and ill shut it off and Sometimes it keeps doing it while other times it will quit and might not do it for a long time. I don't understand how it can just go off and run fine again if something is wrong. Everytime I try to have it tested the light is off and it does not read any codes. It also runs like complete **** when it starts flashing? Also second problem is that my low coolant light will come on and my resovore will be empty then ill add coolant to it and I will go off but by the next day it is empty but my radiator its self is always full. I am confused about these things and would like to figure them out and fix the problems without replacing everything that's not wrong. Can anyone help me?