I'd go the basic tune up route, check for cuts or burn marks on the wires (maybe something shifts and ground out when you put load on it?) check the plugs cap and rotor, it's odd it only happens in gear. Codes don't always tell you exactly what's wrong but if its got a light on its def a little finger pointing you in some direction. Tune up stuff is cheap, also check ground on the battery, maybe run a fuel injection cleaner through it.
These are all general things to check when you have an ill running issue, usually when sensors go it'll set off lights, generally when the pump goes it will set off lights, for TPS and O2 sensors but you've already ruled that out.
I've had a bunch of K5s with intake manifold leaks, it draws in air and creates an enormous vacuum leak depending on the how its leaking and acts as if its a fuel issue. It makes em run like total dog ****, and I've seen it enough to consider it a common problem, check for pools of coolant on top of the intake manifold and even if you don't see green but a bunch of dirt stick and finger in and smell it. <-- go ahead laugh it up.
That's all I can think of but I'm literally just throwing ideas out there without more info.
Good luck
