replace values in a matrix subject to boolean condition

Hi everybody!

I am sorry to bother you with a question so simple but
I think there might be a 
better solution:
I have a matrix of size 360x501 where I want to check
the value of each 5th 
column of each row and replace it (and the 6th, 7th,
8th column) by zero if the 
value is less than 1000. I have written a double loop
to do that but that 
requires a lot of time.

Is there a faster way to achieve this?

Thanks,
   Werner