Nissan Serena car going hot and cold??

sounds like you have air in your water system,is confusing your temp gauge,the water leak could be coming from your expansion tank,run your car with your heater on hot,on tick over,look at your expansion tank when your car is on running temp,are you getting to much pressure,you can get rid of this pressure if you know what you are doing,(DO not try if you don't,)you will get scollded with boiling water,when your car cools down it causes a vacuum,and pulls water out of your filler tank,to keep your water cooling system topped up,if you allow this to get empty it will drag air in to you cooling system,take it back to ever did the job tell them you have an air lock in your cooling,easy job to do,it is very easy to leave air in you cooling system if not done correctly,good luck,come across this problem many many times,I think that is all it is,(Could be wrong ,just my opinion)