I have four Colt eight round magazines for my 1911. 9/10 of the time they do not hold the slide open after the last round has been fired. More than one person has told me the springs are known to wear out. Can I replace the spring with something better? What would you recommend? Can I still get a stronger spring but retain the flush/8 round capacity?

You can buy new springs from Brownell's or from Wolff Gun Springs.
They sell several different types and strengths of springs.

Springs do not "wear out" anywhere near as fast as many people think, even if kept under full compression loads for extended periods of time.  It would be particularly odd for four different mags to start having spring issues at the same time.  I'm wondering if you might not have some other issue going on.

Once you get done paying S&H, you might be better off just getting some new mags.  My favorite are Wilsons.

I love Colt 1911 pistols, but I do not like the followers on Colt magazines.

My carry Colt wears a Wilson (with 2 backups).

Just go to Wolff gunsprings and get a pack of extra power magazine springs, for all of your 1911 magazines.

Mite be a slide stop problem. That's what happened with my Gold Cup would not work with a 8 round colt mag at all. Would work most of the time with the 7 round colt mag.