RAM issue

Hi everyone,
I've recently upgraded to W7 and want to install a fourth gig of RAM to improve performance. I currently have 3x 1GB chips installed.
2 of them are Corsair 1GB DDR2 800MHz/PC2-6400 XMS2 @ 5-5-5-12
The 3rd is the same but runs at 5-5-5-18.
The fourth stick which I just bought is the same as the third. But having installed it the PC will not boot. Power comes on, fans are running and the power light flashes on & off. I get no display or bios post. Sometimes the power comes on then goes off then on and off again, like a car not being able to start.
Taking this new RAM out solves the problem and it all runs perfectly again.
I am aware that the new RAM may be faulty however I would like to get some feedback on whether it could be something else that could be causing this problem?

Thanks in advance :)

Mixing sticks is definitely not a good idea. Even if you get it to run, the slowest stick will determine the pace. What you can try is swapping the slots.
1. try matching ones on 1+2 and 3+4
2. try the matching ones on 1+3 and 2+4

Hi Guys, the reason I'm mixing is because Corsair no longer make the 5-5-5-12 sticks so I had to get close as. The only time the PC will not boot is with the new stick in. It works fine with the other 3 even though they are mixed and unfortunately I have already tried swapping slots. Interesting what you say about the slowest stick determining the pace, I checked the bios and the RAM speed is set at 5-5-5-18. Relevant?

Are the sticks all rated at the same voltage? They can vary from 1.8 to 2.2v. If the new one is rated at 2.1v and the old ones 1.8v you may have a problem if the m/b is running with 1.8v.

Have you tried the new stick of memory on it's own?

juice said:

Hi Guys, the reason I'm mixing is because Corsair no longer make the 5-5-5-12 sticks so I had to get close as. The only time the PC will not boot is with the new stick in. It works fine with the other 3 even though they are mixed and unfortunately I have already tried swapping slots. Interesting what you say about the slowest stick determining the pace, I checked the bios and the RAM speed is set at 5-5-5-18. Relevant?

that is not your ram speed those numbers are your timing

If the new stick is compatible with the motherboard (form factor and voltage) and the PC cannot boot from that stick by itself in any slot when other sticks will boot, it must be a bad stick.

Up to you to confirm form factor and voltage and to rotate single sticks and slots to confirm by process of elimination that only the new stick won't boot regardless of slot.

I never heard of stock timings preventing a boot for a single stick.