Cars are designed to have elevated idle rpm until they reach normal operating temperature so, while 2500 rpm sounds a bit high, I wouldn't be worried about that item.The tachometer needle jumping up and down is a legitimate problem and obviously it's electrical. I would suspect a source (battery/alternator) problem if all the gauges in the car were doing something similar and lights were dim etc. If it's just the one gauge, it seems apparent it is a connection problem. Check all the connections to the distributor and the ignition coil. These two points are typically where the tachometer is connected. A loose connection can cause these exact symptoms.The exhaust problem is entirely different. Your car's exhaust, with it's associated catalytic converter, produce water. This water is seen as dripping fluid from most everybody's exhaust pipe. It is black from coming in contact with soot and combustion byproducts. All of this is normal. However, anything that produces water must make provisions to drain that water. In an exhaust system, that means everything must be mounted lower than everything before it starting at the exhaust manifold. If your exhaust system comes down like normal but then turns up at any point and becomes higher than any point before it, you will collect water there and water forms it's own plug of sorts. Yes, exhaust gases can get by this sort of plug but it does build up back pressure and, depending on how much of a plug the water is forming, it could turn into little convulsions of spewing ickyness.Good luck!