Marvel Unit One behavior

Just getting used to this with only a few trips to the range.
Lower is a Rock River with no mods and zero vertical play frame/slide
Ammo is Eley Orange box (1,085 f/s)
Conversion has the red dot rib installed (but this happened also with the iron sight rib - I think)
Non-lock back aluminum model.

[b]In mag two of four this morning (40F), after (successfully) ejecting round three of five, the hammer followed the slide down to full forward (did not stop on half-cock) on to round four.  I manually cocked it for four and it cycled fine going from four to five.  It also cycled the next two mags after this, one of which was the mag from the failure.[/b]

???

Fairly clean gun - approx 50 rounds from the previous shooting session
Well-lubricated. 
No rough spots or obstructions felt in manually cycling slide other than the hump when depressing disconnector.
Barrel screw was tight on the SS pin (which I now check since discovering the it will loosen up).
Metal OEM magazines.  They say Colt .22 on the baseplate and came with the unit which was ten yrs old but bought NIB.
It did not repeat the next time I used that same magazine.
I already went down one increment to the 8 pound (yellow) recoil spring after this occurred the last session.
Lower has a 19 lb mainspring.
I’ve had zero issues with the 45 upper installed. 

I just read through Bob Marvel's notes that come with the equipment and I have not checked whether my over travel screw is too deep.  This happens frequently enough (once per 2-3 magazines) that I will take a half turn out (adding over travel) and see what happens.  We'll find out fast.


Thanks pistolsmith types.

Ed

Never mind.  I found the answer (actually Travis at FCW did).  Leaving this thread up for the benefit of others.  
My Rock River (and other brands that are built with tighter fitting and tighter toleranced parts) has a longer sear which is for minimizing the severity of something called hammer bounce.  The drawback is that when the tight fitting slide is replaced with a loose fitting one from a conversion unit, the disconnector might not disconnect reliably.  God was smiling I suppose because when I searched the interweb for "long sear" I found that EGW has started making a .005 shorter sear for addressing a problem that was found with some new Springfield gun ( I forgot the model they told me) and though not intended for solving my issue, was recommended.  EGW is twenty minutes away so I picked up the "short sear" just now and dropped it in and symptom is gone.  Trigger however now sucks.  And so did my TF/RF scores in last nights match.

Thank you for sharing the info!  I was mystified, as I have a Marvel and couldn’t begin to figure out what might be the problem. Hope the trigger can be solved.