Drive trim works great on the trailer, or at idle speed in the water. Once I get going it doesn't want to trim up. Im guessing the pump is tired and doesn;t iike the added pressure under power. Any ideas or suggestions before I buy a new trim pump??

pull the nut off the side and check the check ball assembly for crap or corrosion.

what he said, if thats good to go, check the lines

I had that problem, rebuilt the pump, then bought a new one from BAM same problem. Busted the gimble ring and had to change transom assy. Turned out the insulation on the tim limit wires in the bell housing area was failing and shorting in water. Could have fixed it with a can of liquid tape had I known. Try bypassing the trim limit switch inside the hull to test

not trim limit... mechanical indicator and just wired for up and down, now limiter or position sensor.

Is it possible that the lines are backwards? The pump is labeled up and down and they must go that way. The pump will not make enough pressure from the "down" port. If you have it plumbed to the "up" hose, it will work on the trailer and at low speed, but it doesn't make enough pressure to trim while under power. I've seen it, and done it, before.
Eddie

Check what Eddie said, should go up when blue is hot (think blue sky) and down for green (think green earth). If not then you have to reverse both the hoses and the switch wiring. Tabs are just the opposite as the high pressure is for down.

Otherwise can be cylinder leaking internally or relief valve.

Eddie nailed it!! Thanks!! Switched the hoses, trimmed up and picked up 6 mph.. Speed was still climbing when a chunk of driftwood attacked my prop, now to get that fixed.