1996 buick century

[gluck2]

I have a 1996 Buick Century V6 with 56,047 miles. It has been maintained, oil changed every 3000 mi, etc. The car ran great until 189 miles ago.
The first time it happened, the service engine light started flashing. It started making a tapping noise. I immediately shut off the engine and called AAA. They towed it to a mechanic. Pulled valve covers, replaced rocker arm -pivot and stud. (the stud was broken off on the intake side.)
Car ran fine, no oil use, no noise or anything. At 102 miles, it started doing the same thing. Sent it back to mechanic and another stud was broken, another one on the intake side. (the same head) They replced the rocker arm-pivot and stud on the intake and push rod. Drove the car, ran great. no oil use, went 86 miles, service engine light started flashing. sent it back to mechanic. This time it was on the opposite head but still on intake. Mechanics do not know what is making this happen. Please help.
Thanks,
Gary