Hyundai

Most cars have a speed limiter and some come on at 80 mph, most others about 112 mph and high performance cars could be 160+ mph. Engines also have a rev limiter so you can't expect to go 120 mph in second gear.

At first ii used to go 140 Mph on 3gear but now when ii reach 80 its jerks then switchs off

With performance like that I was thinking we are talking about a motorcycle. My data base shows your car as 2000 pounds, 84 horsepower, five speed, 0-60 is 12 seconds, 0-100 is 43 seconds, factory limited top speed 109 mph. Does it have a power chip? Scan for codes, it might have developed some problem being 21 years old. Check fuel filter first.