Just another data point.  I bought a G20 Gen4 about a year ago.  Never had a single problem with it.  Has around 300-400rds through it.  Standard power, high power (Underwood), and all kinds of reloads.  Not a single FTF, FTE, etc.  I like the new grip better and also wanted ambidex mag release because I shoot left handed.  I am very pleased with mine.

I have another thread on a RIA 10mm that took a lot to get it dialed in.  Others don't have a problem with theirs.  So maybe lemons get out there???

I finally finished dialing in my RIA 10mm and thought would report in.  So after filing my slide stop as previously shown, my problem still existed.  I would shoot 3-4 rounds and the slide would lock back.  High power or standard power rounds, didn't matter.  Then by accident I discovered that if I left one round out of the magazine, it would function correctly.  ? I wasn't sure why.  I have a factory 8rd mag and 2 Tripp Research Cobra 9rd mags.  All mags behaved the same.

So I then decided to try what others have done, increase the spring force and also install the square firing pin stop.
I went with the Wolf 20lb spring:
http://www.midwayusa.com/product/121962/wolff-recoil-spring-1911-government-20-lb

And EGW square firing pin stop:
http://www.midwayusa.com/product/510217/egw-firing-pin-stop-oversize-1911-9mm-luger-38-super-series-80-blue

I spent about 20 minutes filing the FPS to fit using regular and Swiss files to finish.

After two range sessions, the problem seems to have been solved.  No more unexpected lock backs.  Tried full power and regular power ammo.  Tried all mags.  All good.

So not sure if the spring for FPS fixed it or both.  Just glad it is fixed.  Great gun to shoot and having a lot of fun with it.

Thanks for the suggestions and ideas!

Well after filing a few thousands off my slide stop and tested, still had same problem. After 2 or 3 rounds slide locks back.  So I compared a stock 1911 slide stop to the RIA.  Appears quite a bit difference in projection.  The attached photos shows .417 vs .461 for the RIA.  I function checked the 1911 slide stop in the RIA and works fine.  I filed my RIA down to .425 and will give it a try and report back.

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I bought my RIA 10mm last month and also have the new rollmark.  Looks like I got new stock.

I have tried different mags, the included (Metalform?) 8rd mag as well as new Tripp Research Cobra 8rd mags (very nice).  No difference.  Tonight I put a feeler gauge between the smallest gap of the slide lock and a typical 10mm round nose FMJ.  A .002 feeler gauge barely fit.  I filed away at least .002 more to increase.  Will see how that goes on next trip to range.

I also have a stock Colt Govt. 45.  I looked over that and see quite a bit more clearance on the slide stop than the RIA 10mm gives.

Thanks all for tips and advice.

After some more Bing searches, I came across this and will try some filing because I have a little copper on my slide stop:
http://forums.1911forum.com/showthread.php?t=374990

I tried different ammo and believe my grip was fairly firm.  I do shoot left handed, so not sure if I am inducing the slide lock or not.  Hopefully a little filing will cure this.

I have an RIA 10mm.  Fun to shoot and accurate, however every 5th round or so locks the slide back.  Tried standard 10mm (Armscor etc) and Underwood. Doesn't make any difference.  What is the best spring to put in this to achieve reliability?  Does anyone have this gun dialed in yet?