My XXX-CR mid conversion didn't have such problem - it was surprisingly stable in air, I just had problems with spinning out of corners. I just have custom made slim alloy heavy chassis.
When I got my X6 and I added weight as it was recommended I had nose diving car as well, I moved part of weight to the rear and is very nice to drive now.
p.s. here is link to my conversion for comparison with yours
http://www.oople.com/forums/showthread.php?t=27655