Well, it could be such a thing that your guitar is setup to handle light gauge strings in standard tuning... then you put these heavy strings on and tune it way down... You most likely have a strat style trem... And since this bridge isn't set-up for heavier strings... it's floating more than normal. Which means that the tension from the strings is greater than the tension for the trem springs... causing an inbalance in the setup of the trem... in turn... causing your strings to go out of tune much faster... That's the most likely... I would think. However... It could be that when you put the strings on the guitar... you didn't stretch them... Stretching the strings when you are installing them helps them break in a lot faster... and along with that. Expect heavier strings to take longer to break in because the mass of the strings is much greater than that of the smaller gauge strings. This means that they have to be exposed to that much more tension before they reach a stage of metallurgy which is, in lamen terms... the "pre loaded stage"... or in engineering terms the apex of the modulus of elasticity... The apex of the modulus of elasticity is known as the comfort zone for metals... they are most stable when they have been loaded with tension long enough to reach the apex of the modulus of elesticity curve.