dell ATG D620 failure to boot on ac power! works fine on battery, whats wrong???

Hi all, i wondered if anybody could help me,

I have a dell atg d620 (which is technically the same as a d620 i am told) i have a problem with starting it, if you turn it on with the ac charger connected the power / wifi/ bluetooth lights flash once and it will not boot at all... screen just stays blank, disconnet the charger and it boots fine!

infact as soon as you get past the dos / dell first screen and onto the xp load screen you can connect the charger and its fine, i have tried 2 new batteries, 2 different new power adaptors, i have also tried new ram sticks and it has recently had a new hdd fitted as this failed. it is running the bext available BIOS for this system (Revison A10 which i have flashed several times now) it has also crashed out a few times once loaded asif sombody had ripped the battery and power lead off just went dead (perhaps only 3 times over the last 6 months)

I just cannot understand what the problem could be now, is there something which could be set up wrong or am i looking at a major hardware i.e. motherboard failure here? any help /. suggestions would be greatly apprechiated!

thanks in advance,

james