You are talking about a nearly 20 year old bike so you'll have to provide far more information for anyone to provide meaningful help. Possible causes are :
Low compression due to ring and/or piston wear
Ignition system issues including but not limited to bad plug, poor grounding, weak stator, bad secondary (coil and or wire) , bad CDI connections
Power valve malfunctions
Float level in the carb incorrectly set
Incorrect jetting
Bad fuel
Dirty or clogged air filter
Bad reeds
The list can go on. Best practice is to verify all the basics are correct :
Fuel
Air into the engine
Compression
Timing
Spark