The gauges bounce a lot anyway but... I did notice, the speedo does bounce with the miss under load too. Hard to explain.
Yes, its a 98... I miss-spoke, it was never a TBI... and was a 305 MPFI then the 5.7 was dropped in and converted to carb/HEI. From what the previous owner (also the ASE Mechanic that did the swap) told be, it is straight HEI.. not connected to the computer. That said, the ECM is still in there. It seems like it is changed over to a more basic set up but, something is clearly not right. He (the PO) said he changed the dizzy and carb twice to solve the issue but neither solved the issue. Not sure if that includes the ignition module or he just changed the lower portion of the dizzy as it has the same cap and rotor as the last dizzy I saw in it 2-3 months ago when I first looked at it. PO was supposed to get the issues resolved but, had no time and I ended up buying it AS IS... thus inheriting the bugs left over from the swap/carb conversion.
Issues:
Gauges bounce
Weird miss under load (heavy throttle or uphill)
A/C not working at all (everything is new) might be a ground issue?
No return line on the fuel regulator
MAF code
Service Engine Light on
ABS light on (all brakes and master cyl are new)
Other than that, the truck has NO RUST (and that is odd for a Hawaii truck). It is HARD to find an OBS RCSB here... overall, it is in decent shape and the price was right, especially when you consider the engine is brand new. Body has 180k on it and all the brakes are new along with the peripherals on the motor. Was worth it to me. Just have to figure out the bugs.
Okay, so you have a standalone ignition then. I was kind of wondering if it was an ignition misfire, but HEI is standalone. I still wouldn't rule out a misfire caused by something. Does it ping at all or have other drivability issues?
AC won't work without the ECM working - the ECM controls the compressor clutch if I remember right. To get that working, you would need to do some wiring to bypass the body electronics while keeping the safety pressure switches.
No return on the fuel regulator sounds like low hanging fruit to fix... I'd suggest doing that first.
MAF code is because there is no MAF and it is looking for one.
SES light will be on forever with the setup you have. You could tune it out using EFILive or something, or just remove the bulb. It will always have faults for things your engine doesn't have (knock sensors, oxygen sensors, TPS, MAF, etc). Seems like your truck only still has the ECM so that it can run the gauges.
ABS light may or may not be related to everything else. It could be caused by the ECM, or could just be a bad sensor or sensor unplugged somewhere. I'm not sure what signals ABS would take aside from the wheel speed sensors.