The OSSC can’t just use 5x for 480i because that would result in something like 2160i/2400i. SNES 480i output has 262.5 lines vs 262 for 240p.
You have picture dropouts on every lineX mode right? Because you didn’t mention it only happens on a specific mode. If you have picture dropouts on 240p lineX2 and not on 480i lineX2 then its most likely caused by the sync jitter.
You could try to set Allow H-PLL 2x to off. At least it reduces the wobbly lines at the top of the screen with the NES. Maybe it will help with your picture dropouts.