my bucks like crazy and wants to stall out at 50 km

without any further information am going to suggest that it's a fuel pressure regulator....hose has fallen off or Techron has "cleaned" the diaphragm clean away so that the vacuum source is suckin' fuel instead of regulating fuel....my other thought was the EGR valve which is supposed to engage to reduce combustion temps and to re-use exhaust gas on deceleration is clobbered with corrosion and not working in a timely fashion~ hope this helps.....will investigate further if either of these solutions warrant further investigation~

without really being able to test anything am going to declare the throttle position sensor is worn or giving erratic signals~ https://www.google.com/#psj=1&q=998+Chevrolet+Cavalier+Base+throttle+p osition+sensor&tbm=shop