How to Diagnose a stating problem on 1998 Dodge Ram 1500 2wd 5.2 litter PU

Just bought the PU a month ago. It was running fine. Park it,and when I went to start it
again it wouldn't start. So first, when I turn the ign., all the dash lights come on,when I
crank the ign. further to crank the eng. I hear a click in the drivetrain area and then
major power lose. Interior lights barely on,Turn key back to off position,and then back to
on and no dash lights. I disconnect the battery for 30 seconds or so and then reconnect
and all power will return, interior lights dash lights etc. when I say dash lights I mean all
the bells and whistle lights such as eng.etc as the run through there motions. Then the
same acures over again. Battery is good, fuses are good, switch the starter relay,thats
good.  Also has a alarm,dont know if it factory or not and it appaers as if fuse panel
under the hood has been changed before.Need Help Please, Any Ideas ?????