Crashes are cause by certain things MTA loads which would be buggy, the reason you crash more in Valhalla is because Valhalla is using functions which crash MTA more than the ones race uses. These crashes are normal, in the sense that it's not your fault. If possible report these crashes (separate each different crash in each report) on the bug tracker. Note that you must be logged in to your Community account, if you don't have one register and use it to login. Then switch 'projects', which is at the top right of the page, to 'new issues' and click 'Report Issue' to report these crashes you are having. Remember to describe when and what you were doing at the time this crashed happened and upload the dumps which can be found in your MTA installation directory (\MTA San Andreas\MTA\dumps). Unfortunately the growth in features of MTA has also introduced new bugs and various crashes, your contribution can help in fixing these crashes.