Skoda Octavia 1.6 SLX (1998) 5 gears manual

I have a issue with the car not running well at all while in reverse, if i push the gas pedal a little too much (slowly, and a regular reverse speeding) it simply jumps out of reverse

Any tips?

Not quite sure what you're saying here.

"Not running well" I'd normally associate with an engine problem.

"Jumping out of gear under load" is very much a gearbox problem, and usually indicates a badly worn gear pair (well triplet since it's reverse).