heat and RPM problems ??

i crashed my truck and busted my rad i drove like 2 days, short trips with it like that, fixed my truck and got another rad, but now my truck dont get warm or hot air unless im driving it, only after around 5 min, and around the 5 min mark my RPM goes up and down until the truck is completly warmed and temp is marked half way, i was told i have an air pocket in the engine, and another guy told me that my head gasket is prolly blown, i dont see how the head gasket ill play a roll in that but any ideas?? pluss the rad fluid i added all evaporated through the reserve tank so i have like under 20% fluid in my rad.........should i remove my thermostat? and run the truck?