Nokia E60 Sim card registration failed

[nyk2005]

hello,
i have a big problem with my nokia E60.
few days ago i received this message "Sim card registration failed".
The sim card is OK(Orange), i changed it, but the same problem, i'll try another sim from another operator(Vodafone and Cosmote)- the problem persist.
I "reinstall" the Symbian soft and hard.
I reinstall the symbian even with Nokia Software update.
Sometimes i have signal but when i make a call i receive "connection error",sometimes it's working(5 maybe 6 hours), but allmost the time i receive "Sim card registration failed"
The phone is network free (unlocked), and the posibility to be stolen doesn't exist because sometimes phone working ok with different operators, and in my country at this moment the posibility to lock phones based on IMEI dosen't exist.
i have the last firmware.
what can i do to solve myselt the problem?
i have a dku cable. What software i need to test what's wrong.
I send the phone to Nokia Service and they told me that is need to replace the mainboard, or send the phone to Nokia Service level 5-6.
other ideas, because the phone is working ok except the voice calls?
what can i replace on the mainboard, jumpers something else to make the phone working?
Thank you very much