phone just cuts off and restarts

[mac901]

ok so i was in chi town enjoying 4g and was amazed by it.but the problem was that while"stock rom also" i was using google maps texting and checking email my phone would just cut off and i would have to cut it back on but before hand it never did this. and once i went backl to 3g it never did that before. not a problem now cause 4g isnt in my area yet. anybody ever had this problem??