Idle too fast
1. PCV system leaking, or have the wrong/failed PCV valve which allows too much air flow?
2. Failed charcoal canister or plumbing allowing too much air flow?
3. Failed or incorrect TBI base gasket? There's a bunch of different gaskets, and some WILL NOT SEAL with certain throttle bodies.
4. Scan tool--look for commanded idle speed
5. Worn throttle body
No low-end power
1. Scan tool--look for knock sensor activity/spark retard, short- and long-term fuel correction, O2 sensor output, Coolant temp sensor reading, etc.
2. What is the fuel pressure UNDER LOAD?
3. When was the last tune-up? Plugs inspected/replaced, plug wires tested/replaced, distributor cap/rotor inspected/replaced, ignition timing checked, etc.
4. Cranking compression test/leakdown test/cylinder balance test--worn rings, leaking valves, retarded cam timing due to worn timing chain
5. A failed torque converter sprag will kill low-rpm acceleration while making little difference at higher speed.
In both cases, you will want to figure out the voltage problem first. Test voltage AT THE ALTERNATOR to see if it's steady and regulated to an appropriate voltage (about 14.2 although it can vary some.) If the voltage is good at the alternator, you need to repair the connection to the voltmeter in the cab. This may mean fixing corroded connections for the entire power supply to the cab--but I'd start with the bus-bar above the AC/Heater system on the firewall.
My '88 has fine voltage at the alternator, but the gauge reads about two volts low (but reasonably steady unless the turnsignals are on.) Cleaning the contacts at the bus bar helped, but didn't entirely fix the problem.