Why is my lawn mower pulsating?

I have had two WeedEater lawn mowers that I have bought from Walmart. They have both had the 500 series 158cc 5.00ft.lbs. Briggs and Stratton engine. Both of these lawn mowers in a very short period of time have started to run really lousy. The RPMs pulsate up and down while you are cutting the grass to the point where you think it is going to stall on the down RPM. This last lawn mower did stop running and would not start. I changed the plug, which was black with soot but the gap OK, and it started OK. The lawn mower had started using more gas for the same lawn. It would run out while I was mowing. When it was just about to run out the RPMs would go up and it would run fine (like it was getting more air and less gas). It seemed to me the lawn mower has been running to rich. A friend mentioned the diaphragm under the carburetor had gone bad. He said it was pulsating because it was getting air and then gas. Another friend mentioned the gas cap. He said it should have vent holes. I took the cap off, which has three vent holes. Inside I fine a cork gasket, round and taking up the whole inside of the cap. It had four vent holes which did not line up with the holes in the cap. It didn’t make any difference if the holes lined up as the cork was loose in the cap and would just spin and misalign anyway. The cork had been sucked outward and it seems like it is sealing off the holes in the cap. I looked at my brother in laws exact lawn mower which he brings to a lawn mower shop to have serviced and there is no cork inside his cap. I don’t know if he took it out or it did not come in the mower or the shop took it out at service time. Has anyone else had this trouble?

Thank you,

TomS