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Old Fluorescent light (two T12)

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Excavatoree:
I purchased 10 (EDIT 13)  Fluorescent luminaires for my basement a long time ago.  They have been discontinued, and to be honest, were pure crap when I bought them.   These have purely magnetic ballasts, no electronics at all, other than a capacitor and what I assume is a bleeder resistor.  One bulb won't start - it flickers for about 10-15 minutes or longer.  It was finally starting, but it keeps getting worse.  I suspected the capacitor, from the way it looks.  It's the most awful capacitor I've seen.  It looks like it has no cover.  There's no labeling at all.  It measured about 2.5 microfarad.

I don't have many capacitors rated in the neighborhood of 220 volts (110V, US system, plus safety margin) but I did have a 2.2 microfarad 400v capacitor.   (A film capacitor in a plastic box) I used test leads to add it in parallel.  Adding this caused the light to work properly, so I guessed it should be about 5 microfarad, and I wanted at least a 250v rating.

I got lucky and found an old internet post from someone with the same luminaire. His capacitor was marked.  I was correct in my guesses, it was a 5 microfarad, 250V AC capacitor.  Digikey was out, but Mouser has one that I think is acceptable.  (had to find one that would fit in the small space below the ballast)

However, I have no experience with this sort of capacitor selection.  Does this part seem like something that would work?  If anyone knows of anything I'm missing, any input would be appreciated.

https://www.mouser.com/ProductDetail/KEMET/C4GAFUC4500AA1J?qs=pqRVuuzkf6beJcsy69%2FR8Q%3D%3D

CatalinaWOW:
I am not familiar enough with electrical code requirements to know if this has all of the proper regulatory boxes checked, but would feel comfortable using it myself.  I didn't see anything in the data sheet that worried me and the stated applications seemed almost appropriate.

108CAM:
The knowledge I've gained from being in a forum with lighting enthusiasts will me pinpoint your issue.

Firstly, the capacitor is only there for correcting the Power Factor, primarily in larger installations where the current drawn by the lights starts to affect the mains installation they're connected to.

In your case, the flickering tube tells me that the tube is going bad and needs to be replaced. You can confirm if the tubes are the issue by taking the tubes from a working light and installing them in the faulty one. If it starts normally, you'll only need to replace the tubes.  Just pop into your local hardware store and head to the Lighting/Electrical department, locate the fluorescent tube shelf and find a matching replacement.

If the problem persists, it's likely the ballast which is failing. It can be replaced but can be rather fiddly with all the wires.
Some photos of the fixture in question would be helpful for further pinpointing the issue.

james_s:
If the fixtures were good quality or interesting vintage then I'd advocate repairing them but since you said they were pure crap when you bought them why don't you just replace them with modern LED fixtures? Alternately you can get LED retrofit tubes that you bypass the ballast and wire them directly to power.

Ian.M:
See https://www.repairfaq.org/sam/flamp.htm

If your magnetic ballast has glow starters, a bad starter can also cause the lamps to cyclically flicker rather than start.  However worn out tubes are far more common so always try new or known good tubes first. 

If you've got glow starters, compatible electronic starters are a worthwhile upgrade, which can extend the life of the tubes and reduces flickering on startup.

N.B. glow starters *MUST* be matched to the tube size and length they are used with, as they must not restrike  with a normal tube's running voltage across them - usually the starter will be marked with the tube size and wattage range its good for, the wattage being proportional to the length. 

N.B.2.  Magnetic ballast fluorescents in good condition emit far far less EMI than any electronic ballast lamp. If you have *any* interest in metrology or low level signals you'll probably be rather unhappy with the increased background EMI levels if you replace them with modern alternatives.

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