Been testing yesterday and managed to work out at least some of the problem. The motherboard with boost on gives the CPU in the region of 1.36 to achieve 3.9ghz default boost. If I disable boost and leave on auto voltage at 3.6ghz it gives the cpu approximately 1.25v which gives max load temps of 62 degrees and which is far more acceptable for my usage.
The only frustration I have at the moment is no matter what I do in the bios the motherboard keeps the voltage on auto. Not sure if anyone has any experience with MSI motherboards, but is there an option I need to enable to stick to manual voltage?
Remember that when it boosts with higher voltages it is because the load is on fewer cores. Therefore the total heat isn't actually worse than when under 100% load.
For example the 1600X and 1800X use well over 1.4v to boost to 4.1ghz. However, because that is a load on 2 cores at most the power draw and hence heat output is actually lower than when running at 3.7ghz on all cores with <1.3v.