Well there are a few basic scenarios I could imagine so lets start with the least expensive. My 960 does the exact same thing at about the same interval you describe. Every couple of thousand miles my low coolant light comes on. I just top it off and don't really worry about it. I believe mine is evaporating through the coolant reservoir cap. I remember reading somewhere that Volvo updated their reservoir caps for this exact issue. I think the newest cap is green. I have a gray cap and there also existst an older black cap. I could upgrade, but a bottle of coolant every couple of years is cheaper.
The loss of power can be a number of diffent things like dirty air filter, stuck hot air flap, worn out spark plugs. If your engine was digesting coolant I would imagine the level going down much faster. To be safe, you could have a mechanic perform a compression check. But I doubt the coolant is related to the loss of power or oil leak.
Good luck.
Justin