Jeffrey, I´ve tried your code and works fine. What might be causing your problem is the speed of addressing logic and/or cpu overclocking. Try changing your logic or a slower clock if it works, then there´s your problem.
A couple years ago a friend of mine had the same problem and he changed the CPU brand and worked fine. I´ve tested your code on Phlilips 87C52UBPN at 11.0592MHz. and HC logic.