1987 Chev S-10

My ’88 S-15 had the 2.5 four cylinder and a single fuel injector that was visible when the top of the air cleaner was removed. This was called “throttle body FI” and I think your ’87 V6 has the same crude FI system. It was prone to clogged FI pintels and had the same symptoms as yours. The pintel design in these vehicles was poor (I have read). You could see dripping in the spray coming from the injector.
I would check this out first because it’s easy to remove the top of the air cleaner.
My ’88 S-15 was curable by switching to gasoline that had correct detergents for FI. The federal government began requiring all gasoline to include the correct additives for FI in the early ’90s.
If you can see the fuel flow from the injector and it appears OK, you would want to recheck your ignition system. FIRST check your connections. Then take out the spark plugs and look for signs of fouling or misfire.
Find a video for ignition tune-up.
It would be nice if you had the correct code reader and a lit Check Engine light but a 1987 GM vehicle is antique as far as codes are concerned. You’re about half way between 1965 technology and a 21st century tech. You may be able to diagnose the problem using the same methods we used long ago. You may be unable to diagnose the problem if you treat it like a late model vehicle.