First off, check the wiring between the tach reader and the EOB motherboard VERY CLOSELY! I've seen a number of them that have a broken wire between the two units, if the signal wire is broken, you'll get the rocket effect.
The fact that conventional works is simple to explain, you're just passing the variable voltage through, even if the triacs were shorted, it would still run that way.
Usually, only one set of triacs is bad, though I replace all of them if I have to remove the heatsink. That being the case, the locomotive will frequently run properly in one direction, but take off in the other.
The Horn and intermittent running suggests a control circuit issue. I'd remove and replace the PIC processor in it's socket to make sure it has good contact, and I'd try a different R2LC.
