HTC One S :: 3G/4G Isnt Stable, Only 2G Is

My HTC one S have a problem of holding the 3G connection stable.

When It finds, and connects to 3G, as it usually does, the internet connection is fast the first 3 minutes, then the internet hangs until I reset the "3G" connection.

The problem only goes away if I switch to 2G, wait a bit, then toggle to 3G again, then it works for another 3 minutes for fast internet, and bam. hangs again. its not "sometimes".. its ALWAYS.. from the first day I got the phone.

If I force it to keep 2G, its super stable. I just miss the much MUCH faster 3G (when it works), but I wont use it until the problem with hanging is fixed.

I think it IS something with my phone (HTC one S) because my sister has same tele operator but with Nokia Lumia 800, and her phone works stable on 3G/4G in the same area as me. (I tried any area.. same problem everywhere I go)

I found some threads on internet, telling its a HTC problem with (3G/4G) switching? . maybe.. I cant force it to use 3G vs 4G ofcause. its a super phone besides this 3G (or 4G, switching) problem.