[H&K 4 LIFE]

First, make sure it is in fact a V1 and not a V9 (safety only). The V1 detent plate will be marked "1+2" while the V9 detent plate will be marked "5+6".

Next, make sure the detent plate is properly positioned. Should look like this (arrow is pointing at the sear axle)

Also, try lightly lubricating the detent plate. Sometimes this aids to smooth function of the control lever.

...What is the lanyard loop insert pin? The pin holding the hammer spring assembly in the magazine area?

Yes, the lanyard loop insert pin holds in the lanyard loop insert, which could otherwise be refereed to as the mainspring housing on other pistols.

 

[H&K 4 LIFE]

Thanks HK4Life! Your pic was very helpful. My detent plate looks OK, and the pistol had been firing DA/SA normally for about a year. The problem is deeper inside the trigger-hammer mechanisms... so off it goes to HK.

Personally, I don't think you need to send it to HK. That would be a good deal of time and money spent (shipping) on a simple problem you can probably fix yourself. It really sounds like a problem with the interaction between the detent plate and control lever.

Have you tried lubricating the plate yet?

If you follow the instructions in this post, it will show you how to remove the detent plate...

http://www.hkpro.com/forum/hk-handgun-talk/142924-variant-1-variant-3-a.html#post1057669

The USP is actually rather simple, and I can think of no reason why the problem you are having would have something to do with anything extensively wrong with either the trigger or hammer components. This is one of those situations where I wish I had your pistol in front of me to inspect.