Why does dieseling happen?

[Mustvid]

Can someone explain the reason dieseling happens? I have a 92 5.0 with box stock AFR 165's heads & Ed Curtis cam plus all the bolt on's you can do. it's got an AOD tranny with 3:73 gears. Timing is set at 16 degrees. I'm running the AFR recommended Autolite 3924 plugs. The heads and cam were installed in the last 45 days and I have been trying to get the tune right. It does not deisel when the engine is cold. Once it's at 190 is when it will deisel. I usually have the tranny in drive and it happens when it shifts into third around 1500 RPM 25-30 mph. It only last for a few seconds. I have taken it to the track and had the timing at 18 degrees and it was real bad at high RPM and WOT. Once I backed the time down to 16 it ran fine at WOT and high RPM. It only does it now at part throttle and low RPM. I have searched many threads on different web sites and I can't find an explanation for while dieseling happens. Someone told me that advancing the timing only effects the spa!
rk in relation to top dead center. Is this happening because of to much timing? Is it because it is to hot a spark or to cold? Should I try a colder plug? I did try running mid grade gas but it made it worse. Once I put premium in it has become fainter but is still there. It is driving me nuts because I don't understand why it happen. Once I can understand why then I will know what I can do to fix it.