Hi, isn't an easy question to be answered ... 
It's seems to me that a process or an interrupt  on Your system is consuming the most part 
of Cpu time:  GUI , telnet and phindows simply do not have time to run .

Because Your machine responds to ping , Your blocking process should have a priority in the 
range from telnet's priority (suppose 10) up to network driver (check it - suppose 21).

An idea to trace  the problem, You can write a process with an higher priority (>max) to capture
the system process state when in such condition,  so You may be able to detect the process 
that is blocking Your system. You can arrage an hardware trigger to start such 'dumper' or
use a sw watch dog trigger if an haedware one is not available.
regards,
m. sangalli