Well, you have exceeded Intels max voltage recomendation, but that's personal choice.
I've got mine running at 4.0 GHz (500X8) @1.280 Volts, 56 C Max.
The only thing I can say is your NB voltage is a little bit low for that FSB your running. I've needed 1.4 volts for mine to stabilize. Also you didn't mention anything about your ram and if it was overclocked.
1.425 seems real high for an 8400. what was your vcore at 4.1 ghz? I have an 8400 too, but i was alway told not to exceed what intel says is the max vcore that you should use, which for the e8400 is 1.365 volts. I think you may just be pushing your chip over its limits.
Well, I agree with RJR: try raising your NB voltage a notch and see if that helps. You are pushing that chip real hard makin it run at 4.5ghz on 1.425v though, it may just not be able to take it.
I can't get mine to stabilize above 3.6 ghz, but i have a crap chip. Be happy that you can hit 4.1!
OK, first remove the ONE stick and leave the matched set in (in the correct slots).
Then go into the bios and set the memory to manual and put in 5-5-5-18 and what ever voltage this ram requires.
Then set the multiplier lower (6-7) so your Cpu doesn't OC to much (oh yeh lower you vcore to 1.3625) and then start at 410 MHz FSB and slowly increase to find out how far you can push your ram. Keep it at 1:1.
Once you know how far your ram will go you will know how high you can OC your computer. Get DDR2-1066 and then the 500 FSB won't be a problem.
If you had two sticks nothing. You should only be using matched Ram sticks (DDR2). It will work in there but the computer won't like overclocking it. Best bet is to only use the exact same ram in 2 or all slots. You can leave it in if you like and find out how far all can be pushed but it probably won't be very good. You'll probably do alot better with just the matched set. Try both and see.