It's odd that you're not getting any other sensor codes. A physical timing issue would usually throw a code about 1 or more cams being out of time and that wouldn't get better as the engine warmed up. Multiple cylinders and it's on both banks. I'd start looking at fuel, air intake path, and electrical issues that could effect ignition coils on multiple cylinders.
The service manual lists possible causes for P300 (Random/Multiple Cylinder Misfire) as:
Poor connection
Ignition system
Fuel system
Intake/exhaust air system
Ignition timing
Injector
The troubleshooting flowchart goes something like this:
Check
spark plugs-> check for intake/exhaust air leakage->check PCV for air leakage->Check cylinder compression->check timing->check fuel pressure
Check that the battery voltage is good. A weak battery could be causing coil misfires across the engine.
Check that there's good gas in the car and there are no fuel leaks. Contaminated gas could cause this. Consider replacing the fuel filter (under the rear seat part of the pump). Most old cars have never had the filter replaced.
If anything has been done to the
spark plugs or coils, confirm that all
plugs are torqued to proper spec and coil packs are full seated on the
plugs. Inspect the
plugs for condition. Inspect the condition of the wiring going to the coil packs for any damage or corrosion.
Check the ignition related grounds are all in good shape and securely fastened.
There's a junction box in the wiring harness on the top of the intake manifold close to the firewall. It looks like a small black cube (GTGJC). That contains the shielding grounds for the ignition coils. Inspect for damage or corrosion. Those grounds attach on the drivers side fender under the cover for the ECM (GTG01).
There's a ground on the back side of the passenger cylinder head (GTG02) thats the ground for coils 1,3,5,7.
There's a ground on the back side of the drivers cylinder head (GTG03) that's the ground for coils 2,4,6,8.
There's a ground on the passenger side of the transmission above the O2 sensor (GTG04) that's the ground for the crankshaft position sensor shield, MAF, and TCM.
The fact that it's happening when cold could indicate a messed up mixture due to one of the emissions systems introducing more air into the intake during warm up. Maybe something related to purge control, variable intake system. Check for any obviously worn or damaged vacuum hoses for the PCV and other emissions components. Particularly the hoses controlling the variable intake system on the back of the intake manifold. Check that the air intake assembly is well attached and sealed. Check for leaks around the throttle body to intake and intake to cylinder heads. You can usually find this by spraying a little starting fluid around the component mating surfaces while the engine is running and listening for a change in idle.
