Weigh in around 180lbs and run between 100 to 105 front and rear. Some guys I race with are similar in weight, but run less pressure. I don't like how the bike feels when the pressure is much lower though. Almost too compliant.
That's quite a high pressure. I'd recommend dropping to 90 and 95 and see how that feels. It may also be faster - better the tyres absorb the vibrations than your body does.
If you're having discomfort after 100km, there's more of an issue than a stiff frame at play. Virtually any frame should be able to be set up so that a regular, fit rider can do centuries without a great deal of discomfort. I've done several rides over 200km, and a few over 250km on my Cervelo s5 - ending tired but not beaten.
- Lower pressure
- Perhaps, wider rims. Wide tyres on narrow rims could be causing the strange handling you're getting at lower pressures. Then lower the pressure some more.
- Raise the bars a bit. If you like being low on short rides, just use the drops more and the hoods less
Edit: I converted your 180lbs to measurements I understand. Depending on your height, 80kg is fairly heavy for a cyclist. Unless you're tall, could losing weight be an option? That would reduce strain on your body in general. Of course, if you're just tall, ignore this.
The pressure values are what I run and am comfortable with them and don't have bike discomfort like the RP. I've settled on those values after testing various other pressures and have found them to be a sweetspot in terms of comfort and bike feel. In some instances, I'll drop a bit of pressure if the roads warrant.
Regarding weight, I'm 190cm/6'3" and like my tire pressure, I'm comfortable.