My 78 started running rough when I drove it for the 1st time in a few weeks.

It starts normally. Runs rough even after it warms up.

Anyone else have this issue on an HEI Vette?

My first questions would be, How old is the gas, and What shape are the plugs and wires in? 

The gas is less than a month old. 

The plugs & wire were new when they rebuilt the engine in 2018.

I have only driven it 5-6,000 mile since then.

I was going to check the plugs but I have aluminum heads and the drivers side plugs won't budge. I don't want to strip the threads.

I did check #6 on the passenger side it it was perfect.

Hey Len,  does that distributor have that electronic module in it that can go bad?

Len, when you say running "rough", what exactly are the symptoms - rough lumpy idle, lack of power, coughing, backfiring?

F4Gary mentioned the ignition module.  Last year I had a coil fail on our '78.  It started fine and would run nicely with no load on the engine (revving the engine while in neutral etc), but if I attempted any sort of incline it would barely pull, sounding as though it was running on two or three cylinders.   

It may be worth pulling the distributor cap and checking its condition though.  

You may also want to check the choke mechanism, ensure that it is coming off when the car is warm?

Let's know the detail👍

It does Gary. It could be the problem. Usually when those fail the engine won't run though.

But it is more like it's miss firing on a cylinder.

I was going to check each plug but the ones on the drivers side won't budge. Since I have aluminum heads I did want to risk damaging the threads forcing them out.

I checked the choke and it is opening correctly.

The roughness is throghout the range of driving. It's more noticable at low speeds.

It almost seems like it is a miss-firing cylinder which would lead to a bad plug wire.

Since I can't get all of the plugs out I was thinking about hooking my inductive timimg light to each plug wire to see if 1 wasn't firing.

Does that make any sense?

Well, connecting the timing light is something I considered on ours once.  I have to assume it would show a misfire, so worth a try!👍

I will post the results once I get it done. Maybe that will get us to a solution.

Before your timing light post, I was thinking of one of those gadgets you hook up between the plug wire and the plug that lights up if there is a spark to that plug.