Applications r not stalling whatsup received pictures not in gallary

Since few days ago my samsung mega 6.3 is not allow to install applications whenever I try to download application from Google play suddenly stop from downloading and my what's up received pictures are not transfer into my gallery if some times transfer so it take 1 or 2 days and sometimes my mobile automatically reset and remove every application and working properly and applications are start downloading everything start working perfectly but after 1 or 2 days still some problems are starting kindly a good solution