'94 F350-Stranded twice lately, acted like dead batteries but just installed 2 brand new NAPA 850CCA batteries 2 months ago, & were strong until the other day. I've charged them fully, to 13.6V each. Glow plugs cycle normally, then there's a sluggish sound to the start. The starter sounds normal but sluggish, like low amps. When I add a 3rd and 4th battery with jumper cables, & squirt some starting fluid, it barely does start, and then runs normally. Literally, I have 3 second bursts to start it, or batteries run down quickly. 14.6V with engine running(alternator works, right?). It acts like there's and undo strain on the starting system. Cable btwn batteries & to solenoid and starter looks good. I wondered if an accessory like PS or waterpump or alternator could be locked up & putting a resistance load on the engine, but engine does not overheat (water pump), steering is fine, etc. Where to start? NAPA? Thanx in advance...