the problem just appeared? or did you took the ps3 apart to do some stuff.
the ps3 is a very complicated machine, and although there are schematics for the fat one even then it can be a pain in the ass.
your problem sounds like the power just cuts off when you are powering on the ps3, like a bad connection.
in standby there's only a little current so a bad connection would work ok, but would fail when turning on the ps3.
it can ofcourse also be a hardware faillure, i'm not an expert.