Thank you. I'll try the simple things before looking to a capacitor though.
Thanks, I'll give it a go.
You might want to check out this conversation about power pickup for that loco:
https://cs.trains.com/mrr/f/744/p/289883/3370004.aspx
Simon
Since only these two models, both Rivarossi, are exhibiting this unwanted behaviour, I'd say it must be a design flaw, or related to a design flaw, and that it can be corrected. Your other locomotives don't seem to have an issue with the tracks or your delivered DCC current.
I have the original first release of the Rivarossi H-8 with sound. It was a LokSound originally, but the loaded sound files elicited a quick laugh from me, and it soon had a Tsunami.
I do remember being alarmed the first few minutes it ran on my layout. It balked, stalled, and seemed to have fits with the electrical pickup. However, an examination soon showed that it has a well-designed system, just something weird about how it gets delivered to the decoder. Whatever it was, it went away. I can't recall if I adjusted any of the pickups...probably...or maybe I just cleaned them.
I run that loco about once a year for about a month or so, here and there. Then I put it away. This has been going on since 2006 when I acquired it. It works very reliably. It helps that the plug from loco to tender is so easy to access.
I think I would start by cleaning the plug contacts. Spray CRC or something like it into the plug and receptacle. Clean up any accessable wiper contacts. Inspect the wipers to ensure good orientation and contact. Make sure the plug to the tender is well and truly home.
Failing that, and assuming your track isn't the problem (the other locomotives don't seem to think so), there might be an assembly/design flaw, something like a bad solder or contact point internally that breaks up the feed to the decoder.
Since no one else has jumped in yet:
ASSuming you have adequate voltage and current capacity (amperes) at the railhead, which is what I'd suspect this is if you hand's said you'd checked it, this is very likely a contact problem. The sound 'functionality draws additional power from the track, and if there is insufficient supply the decoder, drawing power in common, may not have enough voltage to activate properly.
Start by cleaning the wheels, checking the obvious contacts between wheels and frame and tender-to-locomotive, and at least take first steps toward cleaning the track (I'd promote some of the methods of 'gleaming' if you're interested in that) to elilminate obvious causes of 'decoder starvation'.
One expedient fix, although scorned by some on here, is to install a 'keep-alive' (you now see why it's called that!) which is a small rechargeable battery or capacitor that stores enough charge at reference voltage to 'see' the decoder through periods of impaired or lost voltage. Usually this is a module that, in your case, would probably have to be soldered to the DC power inputs to the decoder, in parallel with the leads from 'track supply'.
This does not change any issues you might have with DCC signal, which is carried by modulating the full DC track voltage. By intent this provides an extremely robust signal, lost only if conduction from the track is completely absent or blocked (see discussions of why NMRA adopted this form of 'powerline modulation' for its DCC standard).
There are others here who are more qualified to help you troubleshoot and correct this problem, probably including some with distinctive competence with these particular decoders or installs. Unfortunately, with the PM system failed to where it doesn't accept new requests for conversation, you're going to have to wait for them to read this and chime in to help.
I recently made the move from armchair modeller to building a first (trial) DCC layout using NCE for control. Amongst my roster of locomotives accrued over the past several years of dreaming are two Rivarossi 'Allegheny' 2-6-6-6 articulateds that I'm having problems with. Both are fitted with sound and have been run previously (albeit some years ago) on test track runs without any issues, but if I put them on the new layout the sound stutters and so does the loco but if I turn the sound off the problem seems to disappear and they run smoothly. To complicate matters further, one of the locos ran without any problem, sound on and no stuttering for about 30 minutes yesterday but a problem elsewhere caused the layout to shut down and when it restarted the stuttering issue was back. These are my only Rivarossi units, I have Bachmann, BLI and other brands running happily. Does anyone have any clues as to what is the problem and how to cure it?