Slow cranking when hot

This happened after I installed a rebuilt head. When the car is cold it cranks fast and starts up perfectly. After driving and letting car warm up well, then when I turn off and try to start it will crank very slowly and erratic. I replaced the starter, ground wires and new cable too starter. I am thinking something in that rebuilt head expands when hot causing some tightness? Well I would appreciate any help you can give me. Thank you Ken.