Engine Cooling problem
2007 Pontiac G5 4 cyl Front Wheel Drive Manual 67000 miles

First of all...I had my oil changed at my regular place, they used cheap oil. I usually use synthetic..after a ball game one night...it just rumbled and sounded like major water boiling under the hood. Well it smoothed out and run fine after wards. Then I noticed after sitting in drive thru it began to overheat. @250degrees...odd, then again the next day. Would decrease temp after getting moving again. All was fine about a week. I brung it up and just happened to see a little white smoke after first starting..it needed a little water (coolant) ... after these even had it tested under water pressure to see if we could force a leak...no leaks he said, but no lift to look under. This on average now smoking at start...big white puffs but stops after reaching about 150degrees, whats crazy is my car I had before is doing the exact same thing and its a mustang... 98. A car mechanic an I (me being a lady) I said give it to me straight cause I know repair can be ridiculous but besides that have I missed something...he doesn't think by it running well and not smoking alot that its the gasket...but I also have bubbles in the bottle for fill. ???? What do u think?