Right. I have an 87 350 tpi bird, acting screwy as hell.
Had been running and driving fine, no issues. Today I got around to de-crapping the engine bay..getting rid of smog/air pump, cat, coolant bypass, cruise control (didn't work)..as well as putting new plugs in. What a pain, numbers 6 and 8 are..

Anyway, now whenever I take off from a light, all my electronics and lights go haywire till I get up to speed. Voltage is all over the place.. then once I get over 20-30ish,
Everything comes back to normal. No idiot lights are going off, and it doesn't seem to matter on rpm.. once I'm to speed, things work fine. Stopping, things work fine.
Unless I sit for a few minutes, then they start to flicker.

I never messed with my alternator, and my engine stays running just fine during all this, like its oblivious, it doesn't jerk or die.

At low speed, bumps also do the same thing.. radio, dash, lights..etc.. yet at speed (over 30ish), Nada, works fine.

So uh..what? I'm needing help bad, being my DD and work coming Monday..

Perhaps I made a wire loose somehow while tearing things out??
A little coolant did get into my alt (not running, engine cool)..
If I accelerate slowly (thing USSR era mini van), electronics might dim or flash ounces but otherwise is normal.

 

Maybe check all your power lead connections. IE junction block, starter, etc to make sure something isn't loose. Definitely sounds like a loose/intermittent connection.

 

Do the following:
I have literally said this everyone that had electrical problems, lol,

crawl under the car and look at the starter. The terminal that the big fat battery wire connects to has other wires coming off it. This is a junction block. Clean up all those ring terminals and then put it back, making sure its tight. go for a ride and see what happens.

Next you might want to turn the engine on and start wiggling wires and have someone watch the dash and what not. It is a loose connection somewhere.

 

Sometimes after I post or say something, usually the first reply makes me think "yep. I'm an idiot. Not sure how that idea/diagnosis slipped my mind".
Lol so thank you, kind sirs.
My alt tested good, so yeah I'm agreeing that I probably wiggled a wire/ground somewhere loose when I was pulling the emissions out. Probably the starter or engine>firewall strap.

 

. happens to all of us at some point in time.

 

Always good to have a fresh set of ears/eyes when there is something you may have overlooked. I too am guilty of the same