Print shifting then comes back to correct position

#1
I have been working on this machine for almost 6 months now. I've had ups and Down, but getting some results. The biggest print so far 8.3" tall x 6.75 x 4.5 it was an airduct I made. The best move I made, get Simplify 3d for slicing, best money spent. But I am still having problems with the print shifting. I can start a print and everything run beautiful for several layers, absolutely perfect. Then it will offset over approximately .06in mainly in the x axis for several layers, then shift back to where it needs to be and back to printing perfect. It won't do this every print, but more times than not it will. On large prints it may do this 2 or more times, maybe only once. There isn't any exact height that it does it at, it's totally random. I've always gave consideration to the pot's but once you have a good print and no shifting why should I have to adjust for the next print. I have a bigger fan running on the whole machine during operation to help with cooling, seems to be a big overall help. I'm open to any suggestions. I will try and post a picture.

Thanks