The game lag its not bad but its every 5 sec an it only lags for maybe 1sec.
Its like this
...._...._...._....
I will be running down the road an it will frezze for just 1 sec but it dose it every 5 sec.
Have you installed the game on your HDD ???
How big is your save file ???
Do you have all the auto-saves off ???
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Actually i would install the game on HDD (maybe you have done it too...)
I would disable all autosaves.
Would make 3 new manual saves and delete all the others from my HDD. (actually do always manual saves and put the most of then on the cloud - so leave only 3 or 4 on your HDD)
After that i would clear the cache, and reload a save.
----------------------------------try to play without connecting to XBOX live.
if you save file is big enough i would do all the above, but i would re-install the game and the DLC's as well.