I sorta had the same problem today..
went for a drive.. gave it some, then car behaved weirdly after a while.. wasnt going anywhere, and 1st, 2nd, 3rd felt like what you described.
So anyways pop the bonnet, and i could hear a hissin noise..
im like yup a leak somewhere..
Check your hoses going to the bleed valve..
I re did mine.. car is fine!! goes harder