Running error with New Keynesian Model

I am estiamting a pretty standard new Keynesian monetary model with US data, every time when I specify three observables:y pai i, there is a error message:
??? Error using ==> chol
Matrix must be positive definite.

Error in ==> metropolis_hastings_initialization at 52
d = chol(vv);

everything works fine with two observables: y pai.
this is data for output, pai and i.
output is detrended log, multiplied by 400.
pai is the log difference of CPI, multiplied,
i is the fed fund rate
YBook2.xls (24 KB)
inf_inertia.mod (1.78 KB)

Hi, use mode_compute=6. There are a lot of posts dealing with this problem. See also the Wiki