Grizzlyghost

i have exact same year truck but gmc sierra version, and after start up normal at 40psi then after 5-10min it start to go down to 20psi then 10psi, and after 5-10 more min, the coolant will start to get hot passing 210 to 250 and beyond til i rev my engine to get the oil pressure to 40psi for 5-10 min then the coolant temp will go back to normal at 210... i look for leaks, lost of fuild, smokes, change oil pressure sensor, flush the coolant, i still dont know why it get hot, faulty cluster ? bad head?