L1ftmech would be correct on this one
flow is the key , if you have poor flow of coolant going through the vaporizer then sure it will freeze up. Even if the coolant level is ok in the radiator if the flow is poor it will still freeze up.

I would check the flow in the radiator first, if you dont have coolant noticably moving around inside the radiator then it could be a waterpump issue, also the bypass circuit could be clogged up via a hose or a port at a fitting. These things will require you to drain the coolant and start pulling hoses and checking fitting orfices etc.
example: i had a cat gp25k with the mitsu 4G64 engine doing this same exact thing, i checked everything and could not find the cause for poor circulation. Only when i drained the coolant and started checking hoses and fitting orfices i found a buildup of silicates inside the intake where the coolant circulates through it, it had built up so bad it had clogged the intake up therefore not letting the coolant circulate through the bypass circuit. Yes the bypass circuit in the cooling system is the part the LP system uses to get its coolant from.

I've even seen in a worse case scenario where the block was clogged up and the head had to be pulled off and the ports cleaned out.

I know this sounds like a bit much to do but if you cannot find the obvious then you have to think outside the box and start digging deeper. There is always a cause of a symptom, you just have to figure it out and find it ;o)

Good luck