Default Mobile network keeps switching between 4g and 3g,

My galaxy s2 skyrocket(sgh-i727) keeps switching from 4g to 3g. and sometimes 4g dosnt load things but 3g will. system problem or what? and no its not the service out here before i use to get full 4g.

Sent from my SAMSUNG-SGH-I727 using XDA Premium 4 mobile app
My device:
Samsung Galaxy s2 Skyrocket(rooted)
rom: AOSP

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