Hi there,
My 93 Z28 is doing the exact same thing...I've had the car a short time, but the previous owner says he changed the opti (distributor) just a few months ago...I don't know if he changed the water pump and/or gasket though.
Curious...did you find out what the problem was?? A chevy dealer thinks it's the opti...and they are about to change it...including the diagnostic, parts, labor and taxes it's going to be up over a $1000. So...I'd love any info you might have...
here is a vid of what mine is doing..
http://www.youtube.com/watch?v=lr07LyqzZPA