S500 Erratic Oil Pressure Gauge

I have a 1994 S500 with 160K miles on it. Today I noticed the oil pressure gauge needle jumping around. I have plenty of pressure when moving, but when I lift my foot off the accelerator to slow at a light, the needle goes towards zero and at around 10 miles an hour the light comes on and then dives to zero at stand still and the light goes off, it stays at zero until I move again. Car seems to run great.
Any Ideas?

put a pressure gauge directly on the oil pressure port.

if pressure is constant, replace the sending unit.

Thanks for the help. When I got back in the car, everything seemed normal. So I am not sure what to do now. Maybe wait until I see it again. One other thing, when I saw this problem, I had the A/C on and the temp gauge went up to 100C. All this was happening at the same time.

Thanks

The all idle very low, but I have never seen the light come on even if the needle is below one. Just a slight increse in engine speed will make it jump up to almost peg at running speeds. I have been told this is normal almost since new (92)

Thanks Peter. Today I monitored it a little more. It happens when it is warmed up. I have plenty of pressure at speed, and goes to zero when stopped. The light did not come on at all today. I know this is somewhat normal, but the light scared me yesterday. It seems to run great, but I noticed it is running a little hot in traffic (100C) I will check coolant levels. I am not sure what is normal on these, as I just got it a month ago. I thought it ran at about 80C before, but it is getting hotter in Iowa. Great, Great Car though. Makes me feel like a million bucks. I just hope I don't spend a million keeping it on the road.

I got my 92 in 94, Other than a few major items it is a great car. The AC tends to fail, budget about 3/4 K since dash needs to come out. The closing assist pumps are notorious, as are the BOSE amp. The rest is maintenance, control arm bushings, lower control arm bushings, load leveling "accumulators". Alot of info on this link!
http://pages.prodigy.net/jforgione/MB_S500.html

Peter, Thanks for the advice. I know about the A/C. My friend went through that with his 94' S420. I only bought this thing for $9K, and everything works, and is clean as a whistle. I just cross my fingers, but if something fails, and at the price I bought it for, if I need to put some money in it, I am still ahead of the game. It beats spending $15K on an Impala or something.