A few things to check, after purchasing/installing two new push rods, two solid lifters, and remove the spark plugs:
1. Turn engine by hand to bring valve at full lift, one valve at a time,
2. Make sure coil bind does not happen, this means the coils in the spring do not touch each other,
3. Make sure there is plenty of room between the bottom of the valve retainer and the head's valve guide,
4. Check the strength of each spring on a spring tester, could be weak due to miles and overheating.
Some of the problems could also be weak valve springs, too high of rpm's, piston and valves kissing each other.
Others here may have some good suggestions....