I have a 1992 929 with 115,000 on it. I am experiencing a high idle(2000 rpm) and at the same time, the temp guage is highly erratic. It slowly moves from cold to mid guage, to the top of the gauge them back down to cold and at the same time the idle is extremely high. What is this, as I have replaced the coolant temp sensor. This fixed the problem for a few days but the problem persists.

You could have an electrical problem. Ie loose connector, but it also could be a bad Idle Speed Control Valve (ISC) and thermostat. Try this first Remove the electrical connector to the ISC valve while the car is running if the car idles down to normal then you have a ISC valve problem. The purpose of these valves is to introduce a computer controlled vaccuum leak on start up, while the engine is cold the rpms will be higher than normal to help warm the car up fatster. After the car is warm this valve will maintain around 700 rpm. It also increases/decreases rpm when you increase/decrease load on the engine at idle. You can locate this valve by following the air intake from the filter it should "T" off about a foot up to a 1" line which goes to the ISC valve. There should be coolant lines going to it also. As for you thermostat remove it from your car and check it for proper operation or just replace it these are cheap. To locate this follow the lower, yes lower radiator hose to the engine there you will find the thermostat housing.