I had a problem like that with my first car ('78 Monte Carlo <img src="/forums/images/graemlins/kewl.gif" alt="" />). It turned out to be corrosion in the battery cable where it attached to the starter. Everything was fine when it was cold. But the resistance increased when the cable warmed up (engine heat, hot day in the desert... even juice flowing through the wire) The problem got progressively worse each day. I replaced the battery, alternator and the starter, only to finally realize it was the cable. And that was only after I cut back the insulation. The cable looked fine otherwise. I can't guarantee that's your problem. But it's something to check out if you haven't already. Might want to check out both cables on that note...
1997 Sportage 4x4, auto-trans, Warn manual hubs, 4" UPYOURKIA front lift, TJ 106AA rear springs, 2-5/8" body lift, 31x10.50 treads, SmittyBilt SRC front and XRC rear bumper, swing-out tire mount, OBX LSD front diff, Track Finder rear locker, 5.38 R&Ps and... really crappy gas mileage! <img src="/forums/images/graemlins/shiner.gif" alt="" />