Sounds like a case of bad capacitors on the motherboard.
Checkout badcaps.net for pictures of what to look for which is bulging tops (they should be flat), brownish ooze coming out of them...http://www.badcaps.net/pages.php?vid=5
Had a look at the link and pictures on bad caps. none of my caps look bulging or leaking, although some look as though they been bent over by rough handling ie ones near connectors etc. Ive had another go at re seating the ram and brushed out all the dust as its second hand machine. machine been on all day and no probs so far so im hoping ive accidently cured the prob. Maybe im kidding myself but time will tell.
Thank you for your help and insight, would never have thought to look at caps.