[Hozer 88GTConv]

I think your 160 thermostat could be causing you more trouble than you think.
Since it is basically open all the time your water just continues to flow throughout your system, thus not having sufficient time in the radiator to cool.
As your system has lost efficiency over time, it is finally catching up to you in the form of slowly increasing water temps.
Perhaps all you need at this time is a 180 thermostat. You will probably need a radiator in the near future but maybe not right away.
At least it's worth trying before you drop the coin for a radiator.