1992 nissan 240sx with 230k miles, help needed for another 100k

[tdk]

ive had this 92 hatch for about 7 years now, and am the 2nd owner. i got the car when it had about 150k on it, and im loving it everytime im in the cockpit. when i first got the car i changed the waterpump and clutch as both were going bad.

i just recently gave her a major tuneup, replacing all liquids, spark plugs and wires, new tires, new rims, new radiator, new thermostat, new crossdrilled rotors and brakes.

let me first say that shes been running like a champ and i hope she continues on. my temp gauge is on the fritz as it only works some of the time. i have no idea if its the thermostat or the cluster itself. mind u i just replaced the thermostat. now this isnt really what scares me. i was driving home from a 3 hour roadtrip this past weekend when all of a sudden i heard the engine start sounding like it was working harder. it sounded like it was struggling some what, but it was still accelerating and pulling fine. i didnt think anything of it as i was almost home, but it did it again earlier today. now i want to get to the bottom of this, but i dont know where to start.

sometimes it sounds like its struggling, other times its fine. am i just being overly cautious? the best description of the sound is, imagine towing a boat behind a truck going uphill. help me please.