My bike unstable?

[bikereurope]

i have a 1991 xt600 3tb.

I have never ridden any other bike (since learning many years ago) and therefore i have nothing to compare with.

When i drive my bike in corners and around in speeds under 70-80 kmh its stable and predictable.

when I'm driving on the highway in 80-90 kmh it doesn't feel that stable. its okay when i do slow movements, cornering etc. but if i do small and hard movements on the handlebars (like movements so hard that i wouldn't normally do them )the bike feels unstable. the bike doesn't go into a wobble, (whitch i have never experienced ) but i think this could be a pre stage to one? the bike shakes between my legs. Im afraid that the bike would be unstable on me if i needed to do a quick turn at high speeds due to deers, cats etc.

i the tire pressure is ok

i have a big box on the back, its has only 3-5 kg of luggage,(oil, chain lock etc)

i changed fork oil last fall.


new front and rear wheel bearings last year.

for all i know this is how these bikes handle?

any pointers? is this how these bikes drive? or should i check something?