I'm not experienced enough to be sure on the network issue, but it could be a sign of instability (not necessarily CPU, you're also overclocking your RAM and North Bridge chip too). If you can run Prime 95 torture test without failure for a while @ 3.8, then it's something else, but it's quite possible its just unstable.
When I went over 400Mhz on my board I needed to increase the NB voltage. Also make sure you're RAM is capable of the higher frequency, e.g. if you have DDR2-800 RAM, at 425 it's running as DDR-850. Things like that, I got DDR-1066 RAM so I didn't have to worry about it.
Sorry I can't give any definate information. I've only done a single 45nm overclock.
What I have picked up though from reading:
- keep VTT under 1.4V (I run 1.38)
- keep PLL under 1.6V (or might be under 1.7) (I run 1.5)
- Vcore under 1.4 is safe, under 1.5 is probably safe, some people go as high as 1.8 but wouldn't recommend it (I run 1.52V load, 1.54 idle)
- adjustment of GTLRef can be the make or break (I run 0.67, any other setting causes instant instability)
- NB voltage should be played with (I run 1.5 and it has no issues with it
- make sure you're not asking too much from your RAM
All voltages should be as low as possible. Mine are the lowest my setup would do and stay stable, but I think this is due to the quality of motherboard (Asrock cheapie) rather than what's needed.
When I was stepping up (on mine, can't say it'll be the same for you), Vcore made a initial difference then NB voltage got me higher, then Vcore again, then GTLRef/VTT adjustments.
I'm stable at 4.37Ghz now (460 x 9.5) on the e8500 e0. You should be capable of 4 but it's a bigger overclock than you think on an E8400, its 33% faster than stock, so no suprises you need to fiddle to do it.
NB: ramble above isn't pro advice, it's based on what I've read and my setup, for best advice check with overclocking guides on the net or wait for some of the more veterans to post up. Trouble with overclocking is every system is different, so a lot of it is experimentation and hard to get solid answers sometimes.