94 Blazer, 4.3L Vortec Engin runs fine for a

dirty fuel injection and air filter

If you have replaced the fuel filter this year, the next thing to check is fuel pressure with a gauge. GM vortec engines require 60psi or more to run.
If the check engine light is on you should also have the computer scanned for trouble codes.

check the fuel pump pressure you might not be getting the amount of fuel your truck needs to keep running an easy way to check is to tap the gas when it starts to bog if it picks back up thats what the problem is.

try hiting the starter with a stick of check if your headgasget is blown. To check the headgasget you need a gasget checker kit they should sel that for $24

bad spark plugs, cam sensor or timing, or you could have clogged cat canverter

Could be several different problems things causing this,Disconnect map sensor and try pushing on gas if map is bad problem will be gone or different,you could also have TPS sensor,Distributor problem,or fuel volume problem,Are there any codes set?

Get the codes pulled, often an oil change place has all of the adapters. You need to know what the computer thinks is wrong, before you do anything.

How is it running before it quits onyou. This could be the culprit of an egr valve. How many miles on thevehicle. The egr seems to die on a certain mileage pattern. Also youcould start by cleaning the egr valve before replacing it. This couldvery well help the situation, but this is just a guess. Hope thishelps!!

have 1994 4.3 cpi and the problem I had sounds same.
It was a leaky pressure line in pletium . easy fix $45.00 part $15.00 gasket.
the leak was caused from the fuel lines rubbing together in pletium.
take the vortec cover off ,then remove air regulator in middle and look inside . I turned on power to key and gas spray came from drivers side of pletium looking in hole.fuel line side. if gas spray on passanger side then gas regulator faulty. takes 5 min to check
After fixing truck runs runs great. no more gas smell in cab.
also you can smell oil for gas

its either the idle air control motor or the throttle position sensor. but check the plugs to make sure that they weren't knocked loose and are plugged in all the way. with the IAC motor it will run ok for a little while then it will bog down and only run at idle. with the TPS it can vary in symptoms but will idle high at times