Starting issue

On first start from cold sometimes will not start first time

almost as if something doesn't have the power to turn over the engine,

revs will go up to 500 / 600 very sluggishly and then die to 0

sometimes sounds as if starter is overspinning after start up

only does this on first cold start - press start button 2nd time starts perfectly

I do have a video I can upload but not possible here
many thanks!