Try using a more hot heat range glow plug. Also try using a higher percentage nitro glow fuel with a 4 stroke plug in the engine.
I had the same problems with some rotary valve engines like the Webra T4 series, where they didn't want to run well or idle with my regular 5% glow fiuel, but they ran great with 15% nitro glow fuel. A hotter heat range glow plug might do the same thing as running a higher nitro-percentage in the glow fuel too. The T4 engines idled a lot better with more nitro in the glow fuel. A couple of ASP .61 four stroke engines also behaved the same way too. RCV suggests using a 10% nitro glow fuel with a synthetic/castor oil blend for the oil. All castor might gum up the engine too much but a little castor helps a lot with lubrication and preventing corrosion. I normally use Omega fuels in all my engines or Powermaster, where the oil is a blend of synthetic and castor oil. I have some all castor oil fuels but I use that in lapped piston plain bearing engines though.

I have two RCV .58CD engines myself, but I haven't much of done anything with them yet. I am planning on a twin of some sort to put them on.

yeah pretty much any engine that might have sat unused for many years is at risk of having bad bearings in it. I have had that happen to a number of engines over the years too.