Hi, My 1984 volvo 245 wagon has a pinging problem. When I start it cold, it will have a loss of power and a consistant miss when revving. After a minute, the car is fine with good power. The idle does fluctuate with a random miss. When it gets warm, it starts a past throttle ping. When I give it more gas, the pinging goes quite. I have tried using 89 octane gas to little avail. I need to get this cured since my Cat converter is hollowed out and needs to be replaced to get smogged. I don't want to fry a new cat. What could be my causes? Here is what I have changed:

Spark plugs (Bosch super copper)
Rebuilt AMM (programma brand)
New water pump and cooling overhaul 10 months ago.
Cleaned flametrap (gets clogged fast)

**I do plan to change the valve cover gasket (don't know if it will make a difference. Also, my IAC checks out with the ohmmeter. Are there other tests I should perform on it. If it stays open at all times, could it cause a lean condition. I have the idle screw set almost closed to keep a normal idle range. Thanks, Wayne