Hi there, an intermittent start up issue like that can be a frustrating and difficult problem to isolate and diagnose. I will do my best to help you run down the potential cause(s) and correct them if possible. Could you first tell me what the make/model of the computer is? If it's a custom built computer, let me know what the make and model of the motherboard is.The "99" you're seeing in the lower right is a code from the POST or Power-on Self Test. Certain numbers indicate certain stages in the initialization and test process. The number is ***** to certain BIOS manufacturers so in order to know what 99 refers to, I have to know the manufacturer of the BIOS or Motherboard.
Okay, so the A75A-G55 has an American Megatrends or AMI Bios... so it looks like 99 corresponds to SuperIO initialization. If you have any USB storage devices plugged in when the computer boots, try removing them and seeing if the system will boot consistently.If that doesn't work, open the computer up, grab a flashlight and a needle-nose pliers and look for a jumper labelled JBAT1. That's the Clear CMOS jumper. Move it to pins 2 and 3 (while the computer is off)... leave it there for a few seconds, then move it back to pins 1 and 2. A small "1" should indicate where pin 1 is.
Beyond those two suggestions, fixing an inconsistent boot issue becomes very... difficult because it largely becomes trial and error of stuff like reseating memory, checking cables, switching drives to different SATA connectors, checking the internal USB cables, etc... But let me know if the above suggestions improve the situation any and we'll go from there.