2000 Mitsubishi Eclipse erratic acceleration

So I understand the first reaction to what I'm about to say would be to get a tune up which for my car as I understand it could run me up to $400, an amount I don't have. So I'm looking for simple initial steps that I could do myself.
From 1000 to 4000 rpm's my acceleration stutters and acts as if gas is straining to get in there. This clearly led me to put in the liquid medicine prescribed by my local autozone employee ( fuel system complete care -- for injectors, etc,
) Then I put in an octane booster and a gas treatment. None of which were at the same time. I've had an oil change. I cleaned the throttle body. The only thing that helped was changing the spark plugs and things ran perfectly for like 20 minutes until the car got hot and then everything went back to normal. So I checked the spark plugs to see if anything was burn't or other visible issues but found nothing. Replaced and found car still having the problems.

HELP!
thank you!