CASE HDX 162 running slow when days are very hot

Hi 

I have an HDX 162 Case mower.  I believe it is equivalent to a new holland 1475 or HS 16 or close at least. 

Problem: Machine only cuts properly when it is cool and the day is cool.  As it gets warm machine cuts very very poorly.  Less than 1mph today.  Let it sit until evening. cuts tall feed easily at 4 to 5 mph consistently.  

I have had both pump and motor off and rebuilt.  I have changed the relief valve twice.  I have changed the block that the hoses all connect in.  Have replaced splines in PTO shaft.  Have totally rebuilt knife all new T Rex sections, brand new CNH guards. Can turn the thing over by hand. Everything else is in good running order.  Local mechanics are suppose to come and install flow and pressure guages soon, but havent showed up yet.  

Any ideas??? It cuts like a dream when it is running right but absolute shit when its hot.  Don't want to have to get rid of it, but lots of hay to cut and getting short on patience. Any help would be appreciated.  

Thank-you.  

CASE HDX 162 running slow when days are very hot