Gas Gas ec 2006 1st gear problems

[Munch]

I picked up an gas gas ec 250 2006 and on my first ride my first gear was ok for around 45minutes and then I felt this weird "jolt" when I would give it some throttle in first gear, I guessed something had gone wrong with chain/sprockets so had a look, all looks fine.

I would give it around 20% throttle > jolt > release throttle > throttle again > jolt. That is what happens with slow throttle.

If I rip back the throttle then it will power > jolt > power > jump into neutral.

It seems like some sort of selector/shaft issue because it's only happening in first gear, I tried every other gear and they work 100% fine all through the RPM.

I rode the bike for approx 2 hours after I first noticed it and it didn't get any worse or better, if I ride slow I can ride first gear for a very long time which gives me some hope that the gears aren't mashed or anything.

Any suggestions?