Man, that is a tough one. Sounds to me like your heart rate is high due to anxiety of big races. If that is the case, there really isn't that much you can do other than trying to relax. But that is harder to do than just saying "relax."
There are some thing you can do to try and be more comfortable at big races. Routines are huge. Have you ever watched golf or baseball? Guys do the same thing every time they step up to a big putt or every time they step in the batters box. They do this for a reason. Its a routine and their bodies just take over and do what it is used to in high pressure situations. You can do the same with motocross. Do the exact same thing every time you go to the gate. Think of the same things, visualize the same things, etc. A start in a big race is exactly as a start in your backyard. The only thing different is what your mind perceives.
So my advice is to develop a routine. These articles can also help.http://www.racerxvt.com/category/mental-training