vampire sr1+ slow down the straight

[clemo]

Hi all

got my son a new vampire sr1+ the initial set up was easy and so was setting it,
when we got the track it seemed fairly quick infield but when my son hit the straight it seemed to go the same speed, no boost or anything,
after a couple of laps i upped the timing and acceleration and still nothing, then after about 5 laps whilst going down the straight with about 5 meters to go it took off like a rocket (which at telford is about 40m long)
then the next lap just as hes getting to the end of the straight the boost kicks in and he has to brake hard, which thank god are good!
after 3 sets of batteries its still the same.
hes on full throttle as he hits the straight
its in a b44.2 with a dx3s radio
are we doing anything wrong?
any help would be great
rob.