Digital BW, The Print

I have a 1280 printer, PS6, work in sRGB all the way. I need to print 
a series of pictures at A3 size for an expo, and I really have 
a problem.

My system is calibrated insofar that up to printsize A4, I get on 
paper what I see on my screen. But when I try to print at A3, it seems 
that something in the software gets involved in the brightness. Some 
prints still look precisely like the screen, but others become much 
too light (I sometimes have to set Level gamma to 0.65 to get a normal 
print!).

It's not a paper profiling problem, because when I print an A4 
sized image on those same A3 sheets, everything prints well.

I've tried a lot of things and I don't know what to do anymore. 
Any ideas?

Bernard