1991 lexus ls 400 using water but cant find any leaks heater works intermittenly and engine runs hotter than usual

Sounds like a head gasket/cracked head. Does it not heat when you're sitting at a light but then starts to blow lots of heat after you start rolling again? That's a typical symptom of combustion gasses entering the coolant and coolant entering the combustion chamber. Look for steamy exhaust too, it's another symptom.

It should be addressed as soon as possible, if it were to get worse to where coolant starts getting in the oil then repairs would be much more expensive.

lp