Flashing SES light ???

I just finished replacing the head gasket on my 95 4.3L vortec cpi etc....

after about 2-3 days of running fine.. I got a check engine light and rough running.

Autozone reported the codes as p0300 and p1406...
random misfire and stuck egr respectively..

I know this is indicative of leaking cpi.. and indeed the passenger side of the plenum area looked somewhat washed.. but I had assumed this was still 'clean' from the last time I replaced the CPI.. (about 1 1/2 yrs ago).

I haven't done a fuel pressure check yet.. but I'm pretty sure it is a leaking CPI (again)... but my question really is:

I've never seen the SES light flash.. every time it has come one it comes on and stays on.. but yesterday it was -flashing- at about 1 second intervals..

Is this like the computer 'shouting' at me.. saying I'm throwing codes and they are reall really bad codes??

I've had those same codes before when I originally replaced the CPI.. and it didn't flash like that.

also.. this morning.. the SES light wasn't even on..?? ?

That's all.. any one know what a flashing SES indicates?