Tyre Pressure

morning folks when I got the trophy 1200 the gentleman I got it from ran the tyres at 34/38,and it ran good at those pressures but in reading the manual it’s states 36/41 so put the pressures up, on motorway journey I thought there was something wrong with the bike it felt completely unstable at 70/80 so much so I back off to 60/65,also felt more viby and the mirrors seemed to vibe more than I thought they did,thought I would drop the pressures back to 34/38 and try it and everything back to normal, so obviously the bike doesn’t like the recommended pressures , the bike has just completed a euro tour with the previous owner and the back tyre is a little squared off, would I right in thinking that because the tyre is squared off the higher pressure will be upsetting the balance of the rear wheel