If you're measuring about 2 mA test current in the 1 K range, it's about twice what it should be, so focus on the opamp U3 circuit. The key is to have the 1 volt drop on the reference resistor string, indicated by U3's output being -7.3 V nominal, as shown on the schematic. If it isn't, then there's the problem. If you've already tried swapping all the active parts, then look at the resistors around U3. You may want to especially check the pot R64, and jiggle it around a bit and rotate the wiper a few times. Normally, it should have only a few percent total effect, but if it opens up, then U3's output should tend to stick at -6.3 V instead. Now, this condition should cause the current source to shut off or be a very low value instead - the opposite of what seems to be going on. But, perhaps if the string voltage is lost, it can cause something in the overall circuit to go out of its normal range and lock up or oscillate.
So, first make sure the -7.3 V voltage reference circuit is right, by measuring the voltages all around its feedback loop. U3's circuit operation is independent of the rest of the system, so should be straightforward to diagnose. The reason I always suspect trim pots is that they sometimes take a lot of abuse during their life, being over-torqued and such, even to the point of destruction. Small board mounted plastic ones especially can have their screwdriver slot (and wiper) mangled, or even have their leads broken loose from the board, by overzealous tweaking.
Once you get the right reference voltage, check it while operating through the various ranges and some unknown resistors, to make sure it stays put with different loads.
Ed
