MOSFET Oscillations when bringing gate to same potential as drain
It shouldnt oscillate if there was only a resistance from source to V- and the voltage between V+ and V- was constant DC. 

The situation in practice is another. The instruments and the wires to V+ and V- are remarkable inductive and capacitive reactances. In addition I like to see the wire between G and D (where it is when compared to other wires - a possible feedback transformer). If it oscillates, somewhere is enough phase lag (=slowness) that prevents finding the balance. Fet itself has a remarkable capacitanse from G to D and S and the wires increase the effect.

The situation resembles the shower incident. A man in the shower never gets the water to the perfect temperature when he turns it hot, if it's too cold and turns it cold when it's too hot. The slowness is the flow delay.

Please, show us a photo of your circuit. The wire lengths and routes are interesting. The whole case is interesting and is perfect teaching material.

Try, if the oscillation stops when you connect a 10...100 nF capacitor with short wires between V+ and V- as near the fet as possible. Does the frequency vary if you bend and move the wires?