misfire?

i have what i believe is a misfire / cut out type of thing going on. the truck runs well, except when i am at cruising speed. at any constant speed above 45 mph, i can feel what i can best describe as a jerk, from the engine power going completely dead for a half second or so, then coming back online. this happens at random intervals between 20 and 90 seconds apart. i have added Heet dry gas thinking it may be a tank full of not so good fuel. replaced the fuel filter, checked the fuel psi (53psi at an idle), checked spark plugs, wires, cap and rotor, but i find nothing wrong there. the engine oil and coolant are at the proper levels, and look fine. if i accelerate, the problem goes away, so i can not believe there is anything major mechanically amiss. one other thing i have noticed, since i bought the truck 65,000 miles ago, is that the power delivery when accelerating feels for lack of better terminology, stepped. meaning as the rpm increases, it feels like the fuel injection / timing is being advanced in small steps, rather than being smooth and linear. i have a vacuum gauge permanently installed in the truck, and the vacuum has not changed (which would indicate a leak or a mechanical problem). at 55 mph, it reads a smooth and constant 15 inches, and at 70 mph a smooth and constant 13 inches. which is normal for this machine. there is no check engine light coming on. i live out in the sticks, 40 miles from just about everything, and the thought of being stranded in Michigan's cold wet November is not thrilling. and of course, deer hunting season is 2 days away. and i will be traveling 70 miles one way to get to the hunt cabin. so any ideas would be welcome.

What year Blazer? The ignition system on most of these years was well monitored by the PCM, and misfire codes stored in memory could be a clue. If the fuel system is not consistent, that might slip by the PCM without setting any codes.

Get a tank full of clean fuel before going too deep with the diagnosis. It seems that you've already checked all the usual suspects.

ITS A 2000. and i have put 3 more tanks of fuel thru it. its not getting worse, thankfully. my brother had a straight 6 ford truck that acted similar years ago, before all of the computer sensors. and the distributor bushing (top) was worn bad enough that the reluctor gap was fluctuating enough that it would misfire at random intervals. my truck only has 140k on it, so it seems unlikely. i did notice an aluminum hear sink near the ignition coil. does that need to have some sort of grease or thermal paste on it to cool properly? and what does it (the heat sink) do??