2000 Chevy Silverado excessive cranking before it starts

Sometines when I start my truck it cranks for about 20 seconds before starting. I changed spark plugs and wires, and fuel filter. I had it scaned and only code is the one about a short or loose connection in the gas tank fuel level sending unit. My fuel gauge goes from full to empty when I fill the tank and untill it gets down to about halve a tank reading. What can these problems be and how can I fix them?