it sound like a heater core problem or you have air in the coolant

 

i agree with blkdodge360. you may have gotten an air pocket in the coolant when you changed the thermostat. the original temp for those things are 195 degrees. but your heater core is either going to be behind the dash or mounted to the firewall. it looks like a mini radiator with two metal tubes running from it. if by chance that it is clogged, its a cheap fix. you can get one for under 50 bucks at your local auto parts store. as for the replacement im not sure how that goes. i havent had to replace mine yet, so i dont know. maybe someone on this forum can give you some info on that.