give ur opinion in this 1100 nw problem

sm days ago i got a fone 1100 that was having a problem that it looses the nw after sm time.
then i tried to reppair it. i changer antenna switch, pfo, gsm filters. but still it was showing nw only for 3 to 4 seconds then nw gone.
i flashed it and wrote gud pm also but same problem...
then i was gone out of station for 10 days nd wen i returned the customer came for the fone nd i refused him that it can b repaired,,,,
he said that he vl collect the fone next day.
the pfo from the fone was pluked out by me.
so i fixed it again. but after that now it is not loosing the nw butttttttttttttttt
last day wen got a call from my frnd, nd we talked for neasr bout 5 to 7 minutes, the call suddenly got dissconnected nd i tried to make a call but it showed error in nw connection..l then i made manual search nd selected hutch, but still same.
then i restarted the set now again it is working but i know wen ill make a call for 4 min it vl do same thing.........
it doesnt happened for first time........... i hav face this problem before in same set before going out of station....
now brilliants tell me what shud i dooooooooooooooo..........
i messed with this fone for more then a week..
now nw stable but on making a call, it vl be disconnected after 4 or 5 mins........
waitinggggggg brotherss. plz hlp