94 roadmaster lt1 overheating

[Otakon1983]

Hello all. I have a 1994 Buick roadmaster sedan with the lt1. I was letting it sit and warm up one day and noticed that the temp gauge was getting a little bit higher than i liked it so i shut it off. When i popped the hood the waterpump was leaking from the seep hole. So i drove my truck to work. I replaced the water pump and bled all the air from the system and now the car will get up to operating temperature and will sit there steady for 20 30 minutes and then it will start slowly rising in temperature until it gets too hot and starts boiling over. I can't figure this out. I replaced the water pump, the radiator, the overflow tank, all the hoses heater and radiator, new thermostat, new fan and fan clutch. When none of the above worked i had the heads pulled and checked. The heads were perfect. They were magnafluxed and checked for trueness. The block was also checked while the heads were off. The engine still had the crosshatch marks on the cylinder walls and no carbon build up what so ever. And of course no cracks. So we replaced the headgaskets and put everything back together with all new parts as listed above, the water pump was a reman, not new, but it still does the exact same thing. I am stumped. Anyone else have any problems like this? Maybe i'm just overlooking something. I have been told that the water pump impeller may not be curved the correct direction but i don't want to take it all back apart again if i'm just forgetting something. I need help!