In my experience if you have everything set up properly the loop function in rekordbox DJ is rock solid.
My first question would be about how the tracks are drifting apart – is it the beats (e.g. galloping horses), or are the tracks still beatmatched but it is the phrases that are going apart?
Secondly, how much is rekordbox doing automatically – are you using sync or manually beatmatching yourself, is quantise switched on, are you using autoloop rather than setting the In and Out points your self.
And finally have you checked the beatgrids on the respective tracks are properly set (by using the metronome in Export mode)?