Cuts off

[Greg41]

When blades engage and i start to cut grass going up a hill or somewhat thick grass the engine slows down and cuts off.. it runs fine after cranking but once I get started the engine slows down n cuts off . If I'm just sitting still sometimes it will raise up and down till it cuts off.. I've changed air filter and spark plugs what else could I check

 

[bertsmobile1]

Would you like me to write 15 pages of possible causes that it could be but are totally irrelevant to your mower or would you like suggestions that are highly likely to be the problem so not waste hours of your time
If the former, tough luck, I just don't have the time
If the latter then kindly provide your mower model number plus the make & model of the engine fitted.