Hello Andrew, I was kind of cruising the questions on the parts I helped develop and introduce, the THS3217 being one of them, You are probably done with this issue by now, but just in case ---
Your issue was probably the C182 27pF cap right on the output of the D2S stage - that is always a problem for wideband amplifiers (covering that in a Planet Analog article showing up in a few days -early March2019. The way out of that is to design a low insertion loss filter where the first element is a small series resistor. The tool I used for example Figure 92 in the data sheet is the free version of the NuHertz design tool - it is the only one I have found that allows you to imbalance source and load R's before it generates LC values. You don't need perfectly matched filter designs and in fact those stress the output stage HD wise with their -6dB insertion loss. Not sure you want that 150ohm direct load their either.just going to hurt HD. Unloading the D2S output may move it lower in phase margin, but you might try a little higher value and see if works ok - say 499ohm.
