No service issue and error with wiping cache partition

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http://forums.androidcentral.com – hello i am using samsung galaxy s4 gt-i9505, i am having weird problem. i rooted my phone twice and then unroot again back to normal. i once installed TWRP recovery but now i am back to official, now i am loosing signal randomly, i am using 3 network uk but when i loose my signal, i have to turn my phone off for about 5-10 mins to get signal back. please anyone help with this. and one more thing when i enter in recovery mode it says "no command" under that green android logo, and it doesnt let me wipe my cache partition, when i press that it says "error" and phone reboots (HowTos)