I started using the trim with my keyboard (don't ask me why), but quickly changed to the trim on my MS Sidwinder P2, because it is a thousand times more effective than the board.

Thanks, Nicecloud, I will try the keyboard settings. I have reinstalled FS9 to see if changes I made previously have caused this problem. I don't believe it has always acted this way. When I first loaded it, several planes could be flown at cruise with just altering trim, and could be left flying by themselves for 15 minutes or so and did not gain/lose much altitude. What really perplexes me is the fact that trim has to be altered on the runway before some planes will even take off. Before takeoff it's hard to determine the trim adjustment except for the Cub which tells you a number for trim settings.
Thanks,
Mike

Yes, I have taken some lessons in FS9. I failed my solo because I spent the whole time fighting with the trim. Couldn't maintain 1500 ft. altitude because she kept climbing. I will have to re-try it now. My yoke does a little bit better job in trimming, but sometimes you run out of nose up trim and have to use keyboard to adjust.

  another suggestion is that a lot of aircraft have specs as to what cruise speed is and once airborne and at desired altitude reduce air speed and then trim out.
This is what you do in the real world. Or you set auto throttle to hold a speed . If you are using real world weather you might want to try pressing ctrl z twice to check the wind speed and direction as this can cause a plane to bounce alot when the wind changes from a head wind to a tail wind or a strong cross wind.

Hello Paddlefoot,

You mentioned previously the cub, so let's fly the cub. Interestingly the airfile of the cub is a good one. I think it was made by somebody famous for airfiles. I forgot the name, but surely, that is why it flies so well.
For simplicity, I use autorudder. so we can concentrate on this trim issue.
Let's go to MKJS/Montego Bay/ Jamaica/ Runway 07
Press shift/enter once for better visibility.
Save the flight : Paddlefoot/no trim problems.
Release brakes/ slowly advance throttle while holding slight pressure back on the stick, there we go. I hold slight pressure back and very slowly release the pressure on the stick while climbing away to increase speed to 80. By that time the plane will climb on its own and you will have felt that you could release the pressure on the stick.
Above the golf course, I turn left to the boat on the horizon. Let's go there.
At 1000 feet, Push to level and experiment with throttle and speed, keeping the same altitude. I have to trim up to keep 70 with reduced throttle. We can slow and descend to say hello to the waving prople on the boat, regain altitude and back to Sangster.
This flight is good because I have an horizon in front of me for reference in the turns. It's simple. This is my default flight for testing any small plane. Later I suggest downloading  the free cessna from www.realairsimulations.com which is very realistic in flight, hence the flight dynamics are done by  experts Practice is the key here. I enjoyed the flight. Hope you will. The cub has a nice sound.
With the specs you have, the area overland sangster should be amazing. With a fast plane, cross the island to Kingston. If trim problems, jigsawing, the answer should be in all the feeedback you had.
Hope you enjoy like I did
Nicecloud