I wonder how much drag a 105 ft cord has. I agree with AG - unless you have a really beefy servo, don't expect much power.
I would get a spring scale (like for weighing fish) and attach it to the bot. see how much pulling force it generates. I think you will be surprised at how little it is.
Not to criticize but when ever an engineer has a problem, they break it down into it's components and determine where the problem lies. I suggest a methodical approach and definitely post more info - servo model number and specs, bot weight, wheel size, wire size, failure scenario details and so on.