Working out Kinks

[justbinfishin]

I just drove my 1993 Ranger on it's first longer trip (80 miles) after my engine swap (4.0 replacement). It runs very well, with excellent oil pressure and compression. However, while pulling hills the engine will (sometimes) start to stutter/lose power and the temp gauge will climb a bit. It usually runs over the O in "Normal" on the temp gauge, and will climb up towards the A, then cool down and start running smoothly again. I am not sure what this could be.... Suggestions? The engine isn't stalling, but losing power and running rough so I suspect an issue with fuel delivery. It's 90 degrees right now, so I added an octane booster and I'm running some heavy duty fuel system cleaner through it, but since I haven't pulled any long hills since then I don't know if they helped.

I replaced the thermostat (twice, my first new one was bad) when I replaced the engine.

Thank you!