OK guys,here it goes,93 ford explorer 4.0 4dr 4wd. When I bought it the temp gague didn't move,no problem,changed the thermostat but now when it warms up at an idle the temp needle will spike up half way,then go back down,after a few more minuetts it'll spike back up,and go back down. I noticed when it spikes up the top radiator hose starts to suck shut. Could this be a bad head gasket or worse yet a cracked head?! Any insight on this would be great!

 

Check the radiator cap, may not be venting pressure fully from cooling system, could alos be poor circulation through other cooling components like radiator heater core etc.....