Speed weaving?

Just a question about cars at high speed, I have been doing some top speed runs on the Salt Flats map and in some cars once I hit high speeds the car starts to lose the backend and it starts to weave and sort of fishtail and eventually spin or slide. I have tried multiple different aerodynamic and tires combosand I either get the fishtail at high speed with small aerodynamic improvements such as on the 200BX I get the fishtail at about 220-240mph and it just slides or spins, I have tried to add a bit more downforce to the rearend by putting a Nomi GT wing and then the front lifts a bit and the car rotates, so I put on a GTRX splitter as that is the only splitter or lip I can add, then the car just won't pass 210 mph ( I know this is due to the downforce, I'm decently knowledgeable about this stuff). Does anybody know of a way to negate this fishtail effect at high speed? It does it on some other cars too.

 

https://en.wikipedia.org/wiki/Speed_wobble

 

This may help your case as you need a stabilizer.

http://www.formula1-dictionary.net/shark_fin.html

 

With high speed bonneville type stuff I have found the following help a great deal and allowed me to easily get to 380+ kph with a tuned 200bx using vanilla game parts.
- Use race suspension and lower the front of the car to give it more stability
- Use skinner tyres up front and slightly wider at the rear
- Tune the gearbox to have much taller ratios than needed, first gear should get you to about 130kph and 2nd up to about 220kph tha way you can use the majority of power band in higher gears and be able to use less throttle if it starts stepping out
- Tuning roll bars can also help a great deal by adding stiffness to the rear for more grip

That is about all you can do unless you do what atv 123 says and cheat by adding a stabiliser with triangles.