In section 11-3-3-2 of the TR33 service manual, it mentions adjusting the "semi-fixed resistor VR305" to adjust the decay time - I'm thinking maybe the decay time on your congas are too long, such that they never completely shut down - or their output adjust volume is too high. Maybe that will relate somehow to your 55?? In section 11-3-3-6 there is a specs chart for all of the drum sounds, including peak output voltages, decay times etc. Methinks an oscilliscope might be needed to see what's going on with your congas.
Also, take a look at my individual output mod I did for the 33 - there are output points on the board for some of the instruments, and for the ones where there isn't, just poke arounnd the transistor solder points with a small set of computer speakers and some multimeter probes/clips attached to the speaker input plug. if nothing else, it may help you to process troublesome voices individually (like a noise gate pedal on the conga out etc)
Cheers, Graham