1. Too high or too low power values means either your slope is not entered correctly (there is a sticker on the crank that shows your slope value - this must be entered with your SRM software)

OR

the zero offset is not correct (press Mode + Set, backpedal while coasting, wait for the number to stabilize, press Set, then press Mode). The offset changes daily, especially with temperatures. Calibrate the offset EVERY ride. You've got the good equipment, might as well get the best possible data.

2. Erratic values while you are riding mean that the crank sensor is too far from the crankset. Another possiblity is that the transmitter battery in the crank is dying.