Why is voltage gradually increasing from the source of a power MOSFET?
I suspect what's happening is that the behavior of your MOSFET is changing as it heats up.

See these characteristic curves from the IRF1404 datasheet:



I'll just guess, as an example, that your motor operating current is about 10 A. When you first turn it on, Q3 is acting as a source follower, and the drop Vgs is about 4.5 V. After a while, the FET heats up, and the Vgs required to maintain 10 A through the motor drops a bit...If the FET junction temperature increased to 175 C, we'd now be off the published curve, but we can imagine that the Vgs would drop to 4.0 V or even lower. So the voltage across the motor would have increased by 0.5 V or so.

Edit

A couple of other things to look out for:


Your FET seems to be designed for operation around 100 A (based on what they give typical curves for in the datasheet). You are using it somewhere below 5 A (since that is the rating of your power supply). The characteristics of this op-amp might not be very well controlled at this "very low" current level.
Your FET is burning somewhere in the neighborhood of Vds * Id = 3.5 * 5 = 15 W of power. In a TO-220 package with no heatsinking the FET is probably heating up considerably. The FET is rated for 100 A operation, but is probably intended to be used with low duty cycle pulses.
The uA741 is not a rail-to-rail op-amp. The reason you're seeing 9.5 V at the FET's gate instead of 10 V is probably because you've driven the op-amp to saturation. If the op-amp is heating up during operation, the saturation voltage could be changing, and this would also contribute to drift in the FET's source voltage.