How to stop fallback from H+ to 3G data
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.