10 minutes then starts to surge,

Hello Billy, I have a similar problem with my mower. When your engine starts up, the carburettor closes the choke automatically. This causes a rich fuel mix. However, as the engine warms up, the choke opens up, and the engine begins to surge. It is more noticeable with the blades disengaged, as when they are engaged, it acts as a flywheel for the engine, reducing any fluctuations with RPM. This can be caused by several things.

1) clogged carburettor, this can cause an incorrect fuel to air ration, causing the governor(a little lever that controls the throttle/choke based off user input and rpm) to try and make up for this, overcorrecting, and throttling down.

2) The springs on your governor are loose, the tension is not right and there is slack in the system. This causes feedback when the choke opens up, causing it to overcorrect and undercorrect the throttle.

Try hosing the carburettor down with high pressure air, and if that doesn't work, get replacement springs for the governor.

Let me know how it goes!