WindowsCe Application stops after 18 minutes

Question

I am developing smart device application using vs2008 in c# for window ce 6.0 platform

I have interface sim900 modem and avr microcontroller , am receiving data from the both devices..on a wince device ...

I have use two seprete threads (methods ) for modem and avr ... am getting realtime values and I am also able to post from sim900 modem

but the problem is my smart device application gets close after 18 minute... Dont knw why...

so if u knw the solution plz help me...

Details :

framework 3.5.Net

Ram size of device 128 Mb...