97 Villager A/C blows hot when I hit a bump

Intermittent problem. Hit a bump in the road and A/C unit suddenly blows hot air. No change in fan speed. Sometimes cool air returns quickly sometimes very slowly. Mechanic can not find anything wrong with any A/C component or connection - has been unable to duplicate the problem on a test drive. I'm hoping someone out there has had this problem and found cause/solution!