Instability in fortran code, when compile with new version of ifort

New Version of ifort:

$ ifort -v
ifort version 18.0.3

Old Version:

$ ifort -v
ifort version 14.0.3

I'm running a numerical model of tsunamis. The same computer with the same setting of the run and the old compiler runs correctly, but with the new compiler it generates an error in a modeling time:

....

T1=   17.44000     (min)

PROGRAM is terminated
 instability occurs in 1st grid at (j1,k1)=(        1023           2 )

The ifort FFLAGS are:

FFLAGS = -O2 -mavx -ipo -pad -unroll -w -WB -shared-intel -mcmodel=large

Any idea ?

PD: I have compiled the code in several different ways and it always becomes unstable at the same time ..