I think that your gauge has a minor leak which would allow air to enter your fuel rail i.e. when engine is cooling down. Now that you have a sealed system, everything runs the way it was intended it to.

The pegged gauge could be due to fuel vaporizing within the fuel rail when engine is really hot and not running therby raising the pressure . for the same reason.

Did you also experience difficult starting when engine warm/hot?

Qurious what you find.

'89 Raider V6, AT, SAS'ed, Dana 44 front & rear, 5:38 gears, NP 231 T-case.
'97 Montero SR, cold weather package, 4.90 R&P, BATTLEZ suspension w/ 2" lift, JAOS grill guard & under body protection, custom rock sliders.