You're describing two, potentially unrelated, problems.
1» Loss of engine power (engine rpm decreases with throttle pressure)
2» Loss of engine input to driveline (engine rpm increases with throttle pressure, with no resultant speed increase)
The first problem has to be investigated using the service manual (RM301U). Download it, and the Electrical Wiring Diagram (EWD), if you haven't done so already. They are both in the Resources forum, 80 series section. Obviously, you need a fully charged battery and known functioning fusible links, as well as known good fuel, to start.
If you can't find any DTCs, there isn't a failing component or system, so the best bet is to look for a loose item somewhere. Since you didn't mention any electrical warning lights, I assume the electrical charging system is functioning, even when the malfunction occurs. Since there is only air, fuel and spark involved in making an engine run, look for air or fuel restrictions.
– A broken intake tube will sometimes cause probelms and this is easy to find. Remove the tube and look inside while shining a flashlight on the exterior. Bending the tube will expose any faults.
– A loss of engine power usually indicates a loss of fuel pressure. It's unlikely the problem is a single, or even, multiple injectors, because that would likely cause a stumbling engine, not one that loses all power at once. I'd look for something that is resticting fuel flow entirely.
a) There is a vacuum switching valve (VSV) for fuel pressure under the intake manifold (see page EG-306). This valve gets a signal from the fuel pressure regulator to ECM terminal FPU. Unfortunately, the manual is silent on what the FPU terminal does, but it's not the circuit opening relay or fuel pressure relay or resistor (ECM terminal FPR), since those have named terminals and are completely referenced outside of the VSV for Fuel Pressure section. I assume it's the pump. If the VSV is sending a faulty signal to the pump to shut off, it own't necessarily generate a DTC, since it's doing its job, albeit not correctly.
b) the other possibility is the fuel ump resistor/relay or curcuit opening relay systems, but if those were at fault, I'd expect to see a DTC.
For the sake of completeness, start on page EG-230 and work through to page EG-231. There's no need to go through the entire ECM terminal checklist, unless you can't find something obvious elsewhere. All this sounds like a lot, but it's takes more time to write and read it than it does to do it. You only need a DMM, a notepad an pen and some patience. All of the possible sytem components are illustrated onpage EG-209.
Unfortunately, this isn't a straightfoward component failure problem, so you'll have to do a little legwork and report back before you can expect any further guesses (from me, at least).
The second problem is almost always transmission related. Solve the first problem first.
