Oh, you didn't mention that you were using it as an alternator. If you are using the just the rectifier and measuring the DC voltage, of course it is going to drop. Especialy if you increase the frequency. You have not filtering on the rectified output to stabelise the voltage. In a car, the battery usually stabalises the output. If you are not using a car then you will need a fairly large cap if you plan on powering a signifigant load. Right now all you are measuring is the average voltage through the load, when the frequency of the output increases, the amount of time that the voltage actually stays at the load free measure decreases rather rapidily.
It may also be possible that your motor is not capable of operating backwards. Most motors can but the acutal current that you get from them will often be very different than the current that they require to run.