Plextor 8432 eide writing speed problem

[[D-0nE]]

Hi,i have a strange problem,don't know if it's a software or an hardware issue..let me explain
If i burn at full speed (8x),nothing wrong happens to the process,but when i go to try the cd,it doesn't work (it happens with every kind of cd)
Same Cd,same program (i have nero 5,cdrwin,discjuggler) ,burning at 4x...it works great
Can someone help me to figure out the problem?
If it's an hardware issue,where i can get assistence after the warranty?

Thanks in advance