The best thing to do and it worked for me is download the HSPA+ Tweaker from google play store.
I dont know for sure but i think it works by sending extra data packages to trick the provider into thinking you are in need of a higher and faster connection, thus making your phone stay on H+ rather than dropping down to 3g or less.
I had the exact same problem and it is now sorted with this app.
It may use extra data but it is worth it when you need a quick boost, if you have unlimited data allowance you are laughing really.
you can switch the app off within a swipe and a tap so it really is an all round handy app.