I have a early 90's Johnson 150 VRO, and sometimes the tilt trim wont respond while on plane.
only happens while on plane, works fine at idle or low speed.
replaced the pump
replaced the relays under the cowl
checked the fluid level
cant seem to find any loose wires.
switch on the console seems to be good.

its annoying and I cant seem to track down the problem....any ideas?
what are the chances the new pump is just bad?

I went threw two pumps  tell I decided    to make one from all three.......

 Make sure it's not a rebuilt

Throttle overriding the pumps power? back off the throttle a bit and try it. If it works, the pump may be weak. 

Wire up two test lights to the trim motor's wires. One light to the green wire and one to the red. While underway, use the trim and when it fails see if the light is on (up or down).

This will tell you if the electrical system is working.  If the light is off when the trim is activated, move the light(s) to the heavy power wires that feed the relays. light(s) stay on? Then move them to the small green and red wires that trigger the relays and on back to the switch on the control handle etc.

If the light(s) stay on when hooked to the trim motor's wires directly (step one) then there's a problem in the pump. My  guess  is the pressure limit valve is opening up or debris is getting stuck in it from time to time.

You area talking the pump itself not the motor right?
Is there a way to flush the fluid?

Hook up the test light(s) to the electric motor on the trim pump.  IIRC all you can do to "flush" it is to remove the upper tilt ram pin where it connects to the swivel bracket and tilt it down to drain it out the filler plug. Refill and cycle it a few times. Then drain it again like above.