Printer can't hold temperature as set


I started to print Nylon with Taz6 and every time I do it, the temperature will drop from 255-250 to 225-235 after 15 min. into printing.
My bed is heated to 60°C and fans are to 40%

I start these prints when I leave work and sometimes in the morning temperature is back to 255 at the end of the print.
I use S3D as a slicer and I tried different settings but no change.

These problems don’t exist with other filaments, only Nylon.
Also, on my last print there are some extrusion problems due to temperature? (Printed as shown in the photo)

If you have any other questions feel free to ask.

I’m worried with these temperature issues, it seems like it’s a common problem for users (and that seems to be quite problematic for other 3D printers as well, according to the forums and reviews I read before buying a TAZ 6)… and the thing is that I was planning to move abroad and buy one of these houses in Greece, where temperatures will be high for a significant chunk of the year.