1994 Plymouth Voyager engine running hot

after warm up temp hand goes to hot for 20 to30 secs then back to norm, meanwhile heater blows
cold, coolant blown out of reservoir,put 2 new
tstats & 1 new tsensor but no help, coolant is not hot this can happen 3 to 5 times then engine temp
runs normal until vehicle parked and cooled then it starts over again could this be a blown head gasket
or what do you think it is