My lawnmower stalls after running for about 15 secs if I move it. Any idea why?

Its a b&s 4hp push lawn mower. If I start it and stand in one spot, keeps running. If I try to push it, lasts about 15 seconds and stalls. Also, to get it started again, I have to reprime each time, even if it did run for quite a while last time. I have just done an oil change and put in new filter.