94 Sunbird 3.1 1 valve not opening

I just bought a 94 sunbird 3.1 5 speed, with 151,000 kms on it. Car ran great on the way home ( it sat for 3 months) then started to miss a little. Now there is backfiring through the throttle body, and at certain rpm there is a loud click sound heard in the top end. I removed the front and rear rocker covers and fired the car and there is 1 rocker not moving on the rear bank, far drivers side. I removed the rocker and inspected as well as the push rod, all looks fine, as does the valve spring. If I push hard on the push rod I can feel some spring action in the lifter (pushing with both thumbs, very hard, there is some movement about 1/16 - 1/8") While running though it is still pumping oil through the push rod. Question is, I am not too familiar with the 3.1's is it possible that the lifter is not pumping up, but if not there should be atleast a wiggle in the rocker right? As well it should not be pumping oil to the top? Is it possible that I have lost a lobe on the cam shaft? Is this possible with such low mileage, and also to occur in 1 day? Is it possible that the cam shaft has broken? I have not yet removed the intake from the car, I am wondering if I should or just replace the 3.1 with another used one? Any insightful ideas greatly appreciated. Thanks