Is the 170 K upper limit fixed, independent of the range (e.g. 2 M or 20 M range) ?
It could be something like the Ohms current source not able to provide a higher voltage or some additional resistance across the inputs (e.g. a bad MOV, bad Ohms source protection circuit, dirty board (leaking cap, fried bug).
A test worth while may be checking if the voltage reading input is also high impedance as it should. One can do this by charging a small cap (like 1-10 n) to some 10 V and than check how fast the capacitors discharges when measuring the voltage.