I have a 1994 mercury sable and at times it will "lag" when I take off from a stop. what is this caused from?

What i find most of the time in this case is the engine is to lean on fuel and when you open the throttle body valve (push the "gas pedal") you are introducing MORE air and the engine will "fall on its face" just at beginning of acceleration. You might check for a vacuum leak.

I would think the coolant loss problem is unrelated.

How high are you filling the coolant resivor as its supposed to be only about half full not all the way during normal operation.

It is a 12 year old vehicle. You are lucky it only lags. Your slow take off could be a variety of issues. Clogged injectors, loss of compression, plugs, wires, etc.

Odd that it spits out water only when you turn it off. Do you have the proper blend of coolant/water in the radiator?

You need to fill the radiator twice a week, and you consider that 'not much'? What would a lot be? If you want to keep this 12 year old vehicle, take it in and find out what is wrong. Could be any one of a number of things, and I can't see it from here.

Because it is a 12 year old American made car. American made cars are not built to last more than 6 years. After that they are pieces of junk that littler roadways and guzzle gas. Get a new car. Buy BMW.

reset the vacume advance a degree or 2.

I think it is a mismatch of ignition timing which needs adjustment.