454 overheating

I have a 1988 27' class a motorhome w/454 motor. The motor runs xlnt. The problem is it runs hot under load on hot/warm days. up to 230-240 deg. does not run hot at low rpm. Runs at a 200-210 on cool days or flat roads. has new 4 core radiator, new hd GM clutch fan with 7 blade fan & 2 electric fans (pushers). Recent tune, plugs, wires, cap & rotor. 2 new mufflers. Im thinking maybe timing curve & or carb jetting could be the problem. Carb is stock Quadrajet. Timing is: static idle 9 btc total 24btc. With vacum: idle 20. Total 34. vacum at idle to dist is 15. Distributor is stock hei. Are main jets to lean? Will a high flow water pump help? any help would be greatly appreciated!!
Gary

The cavitation theory makes sense. Thanks for the help. Is this a common problem with stock water pumps? Thanks for the help.