I've been running 5-6 days a week now for many years.  No significant injuries. In fact, the last 2 years have been almost perfect.

I recently took  off a month, doing only 1-2 easy runs a week. I'm going to tackle a marathon this spring and wanted a little break before ramping up my mileage. Last week, I started back up with my training. 

On my last 2 runs, around mile 5, my knee just stops "working". Not painful. It just won't go. I don't how to describe it any other way. Both times, I've stopped and walked for a bit then literally willed my leg to go. It eventually does go but it is very odd and more than a bit annoying.

I've been running in minimalist shoes for several years and I'm pretty sure my form is not too bad. 

Does anyone know anything about this?