I have a 1997 Dodge 2500 with 5.9L cummins diesel. I've owned for about 8 months, with no issues, has 195000 kms on her. Hasn't had any upgrades,bone stock. Anywho, I only drive it occasionally and decided to take it on a road trip, drove to my destination and all around my destinations town which by the way is at higher elevations and is extremely hot 32 C.
Went to leave in morning, little cooler outside now. Had no power, started fine and idled normal. But wouldn't accelerate past 1000rpm in gear (automatic). In park it won't rev up past 1500rpm. Disconnected battery for half hour, didn't help. Air filter looks like its dirty, could be the issue, don't know last time fuel filter was changed either.
I was about to leave the truck behind after a couple hours of no luck and no change in performance, decided to move it and it suddenly worked just fine again. Ended up driving it all the way home through the BC mountains for 400km home. With no issues.

I have no clue what happened, checked for codes at home and its all clear. The only thing i can think of is a possible air bubble or something in fuel system and when the temperature outside warmed up considerably after a few hours, it went away ?

Not sure if i should be looking into fuel pump,injector pump, sensors or what. Also now that i think about it, it does kinda smell like diesel when around the vehicle since we got it, but being my first diesel, thought it was normal.

any tips or a guide in the right direction be appreciated, One other thing, not sure if related is that the fuel sending unit doesn't work, hasn't since i've owned it for 8 months.

Don't use any fuel additives either, should i be ? and whats the best for these older cummins ?