How to mosaic to new raster for multiple DEM tiles?

I took some DEM files and turned them into raster .tif files, i then tried to Mosaic them together using the mosaic to new raster tool and i keep getting two problems consistently,

1. there seems to be a miscalculation/discoloration in the top left corner and i cant figure out how to fix/adjust it, it doesn’t really impede me from using slope, aspect, etc. but it looks bad and this is for a university project.

2. i am getting gaps in between some of the raster mosaic tiles (ie. not completely smooth so it looks bad) i tried a couple of the fixes i have found on here to patch them up using the raster calculator but none of them ended up working for me.

i am using arcmap 10.2.2 and the only thing ive done is added the dems, changed them to raster, projected their coordinate systems to match the data frame, and then mosaic to new raster.

Here is a picture of the multiple DEM files in full mosaic with errors.
thanks for looking at my post!