I have a 2003 ktm 250sx and when shifting at high rpm's it won't shift.

Hey Jerry,
No, you do not want a lighter spring.. it will cause the engine to start "falling" out of gear, or not staying in gear when you shift... hard shifting is usually a sign of something wrong.. possibly a bent shift shaft, or worn/bent shift forks.
If it is hard to shift, both up and down, in all gears, its more than likely a bent shift shaft..(the shaft the shift lever is bolted to and runs through the center cases to the right side and controls the shift cam.).. You will have to remove the shifter(lever) and the right side cover, the clutch assy, and maybe a couple other small items..IF you are NOT intimately familiar with this repair - Take it to a qualified professional!!!

see if this link helps you out.... My 2004 chevy trailblazer won shift from second to third gear Google Search

Is this a genuine part if not the part may be at fault also if not is there any dirt through the hole in the crank case the gear shaft goes through a simple solution is to use a multi tool and sand the shaft or crank case hole

Can we have a video of like the chain and sprockets on the bike it could be a motor problem you might be running to low of octane gas, what ratio is your gas, and if you need more help call my KTM dealer (814) 763-2453 they will help you out

Either the shift linkage could be binding or you may need to adjust the clutch.
The other problem might be a synchronizer in the transmission going bad.

It sounds like the end of the shifting shaft has broke loose, it is a welded joint, If you can grab the shifter with your hand and feel that thing slip in there, up and down,that might be the problem, but splitting that case, ugh, total rebuild.Too bad you can't get to it through the clutch. Hope this helps.

It should be around 2000 RPMs. Sounds like it's not going into overdrive. Probally an enternal problem.

this sounds like the selector fork has broken inside the gear box so its a strip down to sort this one out

check your tranny fluid and filter. you may have a clog some where or could be low on tranny fluid.

Correct me if I'm wrong but the 250SX is a 2 Stroke engine with a Gas (1.98 Gals Unleaded Premium 91 octane Recommended) to Oil Mixture (using Motorex Cross Power 2T at a 40:1 - 60:1 depends on oil quality) and when KTM says to Change the transmission oil the amount called for is 0.7 liters engine oil using Motorex Top Speed 4T 15W50. These Quanities are from an 04 250SX could not find a 2000. So consult with your local KTM Dealer's Service Department and pickup a Owners Manual at the same time it would be in your best interest. Liter breakdown 0.7 liters = 0.739681 quart (US liquid) OR 0 quart (US liquid) and 1.47936 pt (US Liquid) / 0.7 liters = 1.47936 pt (US Liquid) OR 1 pt (US Liquid) and 0.958726 cups (US)