I didn't want to say anything because I'm not really certain when it comes to this sort of stuff, but you might want to invest in one of those heavy-duty batteries that are supposed to not get so bogged down when the starter engages. I remember some folks saying that they helped, I think. I think I also remember folks saying they didn't.

So, in conclusion, you should try this thing that's likely to be expensive that I have no firsthand knowledge of and little theoretical knowledge of based on some things I think I remember as being inconclusive to begin with. Good luck!