I agree with madaboutmillie that a lot of horses try to run away on their way home.  My mare used to do this when we got about 1/2 mile from home, its was very scary at the time as she would rear, spin, start to shake then take off at full pelt!  I gradually got her out of this habbit and learned that I needed to relax more myself the nearer we got to home as she was probably feeling my apprehension about it after she's done it the first time.  Do you know if there are pigs on the farm you ride past (Skip has a fit if she can even hear pigs, never mind see them!). 

Not sure what else to say as you haven't given much information but hope you get her out of this cycle soon.  There is nearly always a valid reason for behaviour like this and its just a case of looking for the cause.  Bolting is either from total fear of something or pain (running away is different to this and is often confused with bolting).  When Skip was at her really bad stage and I knew it was about to happen, sometimes I jumped down to reasure her and walk for 5 minutes so that she could calm down, then get back on to ride back to the yard.  I used to think this meant defeat, but after reading Kelly Marks book I understand that it's not the case at all and can give you both the breather required to just calm down and gather your confidence.  We're miles better now and have great fun out on hacks. Good luck hun, I'm rambling now so I'll go.