jremington:
The 1N4148 diodes are definitely not appropriate for those motors as the maximum repetitive forward current is only 500 mA. They may already be damaged. Try 1N4004 or similar.Also, breadboards are not intended for high current (> 100 mA or so) and the contacts may be burned, especially the power rail. Solder all the motor connections.
Oh wow, thanks for the input on the diodes, I had a feeling those weren't adequate.
However, I have now switched them all to 1N4002 diodes, which are essentially the same as 1N4004, but there is no, or little, difference in performance.
I'll take note of what you said about using breadboards. I'll try to make a more secure circuit and see if there's any improvement. Thanks for the advice.