Problem with speedo, help?

[steveconn]

I'm new (again) to R/C cars after a 25 year break.
Raced for the first time last night with my K2 and everything second hand and had a issue that no one at the club could fix. I'm running a Speed Passion Reventon R speedo and it seems to have a problem, it work perfect 90% of the time but every now and then the brake doesn't work, happened twice at the end of the straight and smashed my wishbones each time. Tried changing everything possible with the program card but still happening. It seams to be if I come off the power and let it roll a few yards before I brake, if i come straight off the power and brake it works???
Using Sanwa Exzes X radio but I'm sure its fine as all settings standard.
Any help appreciated...