What Is Wrong With My Lawn Mower. Starts Fine Then Almost Dies Then Runs Fine This Goes On The Whole Time I Am Mowing The Lawn. It's Only The Third Time I Have Used The Lawn Mower?

Don't know what type mower? But I have 3 mowers w/ B&S engines and have to keep the foam filters clean.If that doesn't work pull the carb. Off and clean . I one time bought a mower from Lowe's and used it a couple of times and it started dying I took it back and got another mower,then it done the same thing. Then  I realized it was something I wasn't doing right. We have a lot of dust here. I cleaned the filter and it ran great. Everytime I get ready to mow now I check the filter first.