I have had the identical problem.. have tried everything to fix it.. put new capacitors on the speedie even a spektrum capacitor on the RX..
Just sent the tekin back to Tekin for warranty and had to purchase another tekin RS.. had the exact setup that you had.. but with a 17.5.. this was a very expensive exercise. . as I broke a Exotek Chassis and a top deck.. always seems to glitch at the end of the straight when you tap the brakes.. and you then have no brakes or steering...
Don't know what is causing this problem but it appears there are allot of people having the same issue. I even purchased a different module just to use with tekin as I don't want this to happen again... its not the spektrum either as it works fine on all my other cars.. it just seems to be the combo..
Cheers