89 Suburban cutting out

[87Skier]

I have an 89 Suburban with a TBI 350 motor that is cutting out. It only seems to happen if you drive it for more than 30 minutes. It also seems to surge a bit all the time at speeds around 30 mph.

When I say it is cutting out, I mean that at highway speeds or under acceleration the engine cuts for a half second to several seconds. When it does this, it feels as if the engine has literally died and the suburban loses speed very quickly before it begins to fire again. The worst so far as been when it went from 60 down to 40 before it started pulling again. During this, I didn't move my foot at all. Under heavy acceleration it will ping noticeably while cutting out.

This is the recent work:
Summer 2015: distributor died and replaced with cheap eBay unit
March 2018: Replaced cap, rotor and plugs
April 2018: New fuel filter

I'm thinking it's related to the distributor/ignition, injectors, injector controls, or the fuel pump.

I know the fuel line is flowing fine since the problem never happens when the engine is cold. Yesterday, I drove it for about 2 miles at 85 mph without it cutting out or hesitating, but 15 minutes later, it would barely do 45 mph.

It could be RPM related. If I accelerate very slowly, it will not cut out, but the speed is still limited to about 45. I have a Turbo 400, with 4.11 gears and stock size tires, so this is about 1900 rpm.

Clearly, there is a heat component to this. Since it only happens when it's hot. I have an earlier style HEI distributor I could put in.

Any suggestions?