i'm not giving you any recommendations, only saying that there may be a different mechanism operating than you think there is. the rave says that the speed signal goes to the amp, not to the radio. the pin-outs for the radio don't show anything for speed compensation.
it may be that the amp needs changing instead?

 

Ah, so maybe the radio is doing it's thing and the amp is actually cutting out for a yet undetermined reason, if definitely sounds like the amp popping to me. Maybe with the incompatible radio the speed system is disabled and the cutting out problem isn't occurring. I missed the part of your post saying SLABS communicated with the amp and not the headunit. Now I get it, good observation. I have a spare amp to try however the problem could be on either end of the communication link. What's odd is that the sound will cut out when stationary with the truck in ACCESSORY and the radio on, so I wouldn't think that speed control would be part of it. It also never happened at any time with the replacement radio. Thanks again mollusc.