Difficult to start when hot.
 

I have a 1993 F150 with a 4.9 liter inline six. It starts fine when it is cold. Once warmed up to normal operating temp the only way to start it is to floor it and after 8-10 seconds it will fire and after feathering the throttle eventually idles normally. Any ideas as to what could be causing this? It just started yesterday. I changed the head a few months ago and it has been running fine until now. Thanks in advance for your input.