I would bet it is a misfire.

I suggest the usual cap/rotor/wires and plugs, plus the fuel filter. Setting/checking timing and valve clearance is probably a good idea (not that either of those will cause a stutter, but the 2.6 just seems happier when everything is set correctly.

Beyond those things, I suggest looking at the air intake tube between the MAF and the throttle for cracks. The fuel injectors may need some kind of clean/rebuild/replace attention, if it sat long enough. Replacing the vacuum lines might be in order, depending on when it was done last and what they look like currently.

The diagnostics of a compression test may be in order. It will at least tell you how the engine is as far as the basic mechanical condition.

Other, less common, issues might be the coil or the condenser. Although, the condenser usually causes a no-start condition and the coil is generally functional or not.

Start where you like, but I would bet dollars to donuts the stutter you feel is indeed a high speed misfire.

Michael