Blown Head gasket?

Engine Cooling problem
1997 Buick Lesabre 6 cyl Front Wheel Drive Automatic 70,400 miles

I was driving my car last tuesday when it broke down. I was going about 40mph right around where it shifts, and the car jumped as I accelerated. I would have thought nothing of it but the car didnt continue to accelerate. I tried to accelerate again and the car jumped again. After this second jump the car lost engine power. I was luckily able to pull into a parking lot before I lost momentum. The mechanic told us the problem was the intake gasket and replaced it. It still didnt start. The mechanic then came to the conclusion that it was the head gasket and it was blown. He is calling me a liar about the absence of white smoke and the car overheating. I was driving the car for about 7 minutes and the temp gauge was no where near the overheat status. The car is under a 90 day warranty and he is unwilling to fix it for free saying that it is my fault for not seeing the tempature rising. There is the NY lemon law but I am asking what I should do about the 5000 dollars I am out and/or what could be the problem. I dont believe the mechanic and think he is incompetent.

Engine Performance problem
1997 Buick Lesabre 6 cyl Front Wheel Drive Automatic 113000 miles

it was blowning white exghaust on water in the raideator when I filled it back up there was little white chunks of what looked liked and smelled like oil it has lose of power but theres no leaks anywhere

1992 Buick Lesabre 6 cyl Front Wheel Drive Automatic 161000 miles

i was driving my car I has stopped at a light. I guess my belt had came off due to a fault water pump. I didnt know the belt had fell off the steering just stiffened up on me. I drove the car for about 4 more minutes till I got home. The next morning my car wouldnt start. Does it need a belt to start or is it a blown head gasket?

May have drained battery down if alternator wasn't turning due to broken belt.