if you have enough coolant in the system then a flow check would be the next thing to check. I would pull lines to verify flow, you may find there might be a flow restriction at some point. I ran into this on a cat once and the silicates had built up at where the lines go through the intake and clogged it up there. I had to clean out the whole bypass system. If you find this to be the problem you may want to try a different coolant like extended life coolant. The green is notorious for build up in some systems due to it's high silicate content.