I believe I've narrowed the issue to the probe tip, as you've suggested. Seems hitting it with a shot of cold air will calm the oscillation for a short period of time. The probe tip seems to be an amplifier microcircuit in the P6249 with 4 capacitors. The microcircuit had some red/rust color material on the traces, and I am leaning to corrosion as a potential contributor to the problem. Capacitors or microchip degradation may also be contributors. The tip is very well contained within the housing, and easily destroyed in attempting removal. There is a small, maybe 1/8" hole in the housing covered by a piece of tape. The traces of the circuit can be accessed at this point, with the remainder of the microcircuit well contained in the housing. It does not seem to be easily repairable. I have not been able to make further progress, and am not willing to sacrifice my remaining two probes, as even with the osculation, they are usable at higher frequencies.
