XRAY - Model racing cars

Hi. I've a standard M18. I made some modify, like the carbon upper deck (my rx is a little big and I need more space to fix it) and hexagon tyres.
I run on tarmac (or asphalt?? I don't know witch is the right word) tracks, with some friends. They have the pro version. All with similar motor (mine is the HW 12t 7800kv). They run very fast and with a good precision, but I can't manage mine. When I steer, also if I use very gently the radio, the car is too much reactive. It steers too much and too quickly! I thought it could be a servo problem, so I replaced the old one, but nothing. I tried some modifications on the radio, such as expo and servo speed, epa and so on, but nothing. I had a problem when I brake (the M18 goes tailspin) so I blocked the front differential and it improved the driving feeling, but it overturn always. Could the free rear differential make this kind of problem? I was thinking to buy the ball one... The tyres are 45F and 35R

use some CA glue on the outside wall of the front tires to make them stiffer or use 50 shore front tires.

Hi. I forgot to say that I already glued the external part of the tyre...