Hi Pablo,
What we have here is one very unhappy slot car! We can rule out the obvious, bent wheels, axles, out of round tires, twisted frame and improper guide alignment. Tilting is one thing, hopping and jumping down a straightaway when everything is rolling round and true is something else..............
NO JOKE!
It could be that the whole car is just plain "unhappy" or the car could be "unhappy" with one or more of its parts.
I had this happen with a car that was really pissed off at the Gold Cup guide flag. We tried everything to try and stop the car from just jumping out of the slot for no reason. There is nothing wrong with a Gold Cup guide but this car must have felt it deserved a real Cox guide. When I put a Cox on the car it was fine.
I know you've trimmed the "pontoon" off that guide but............
In the 60's my raceway owner-boss-mentor Leon Pardee would have thrown the car at extreme velocity into the nearby Coke machine destroying absolutely every component. Sounds like you've resisted that temptation!
I think your chassis design is just fine. If it was made from good old brass rod I can't imagine why it would not run just fine. Tilt in the corners with too much tire, yes, but hopping and jumping down the straights, no.
I hope you won't give up on an R&C GP car as the skinny little cars are so unique looking on the track. They can be a real hoot!
Hopefully you will build another GP car but..........call me superstitious........I'd loose that guide
Onward Pablo!