He put new batteries in last week..Alternators are at 14 volts. idling when you push the trims buttons both volt gauges go down to 12 then back to 14 after letting the trim switches go. He also sets the battery switches on all or both. In my fountain idling, if i push the trim switch only 1 gauge drops to 12..and my battery switches are on all also. Having that extra ground connecting the 2 blocks couldn't cause a spike of some sort

So lets eliminate or confirm the battery/ground theory. Get a known ,good battery ,and put it in the boat not connected to any grounds. Connect all trim pumps , trim or tabs to this fresh hot battery. Run the boat and work the the trims and tabs and see if the motor cuts out. If motor does not cut out then it is a ground or battery problem. If it does cut out then it is not a ground/battery problem and something weird like arcing . Also pay attention to whether it does it with trim up or trim down or both. Trim up pulls more amps. These are just cheap things to try and gives us more data. You should not have the battery switch on all or both as this will confuse as to what battery or wiring might be the problem. Also you should not run with switch in all or both position as most stock alternators will not have enough juice to charge 2 large batteries . Some pics of the way the wires are connected to the perko switch and batteries may also help

Chris

Does the switches in 'All' combine the two engine's charging and battery systems with each other on this boat ?

yes

Chris sounds like a great idea so how do i do this...disconnect the wires going to the pumps from factory wiring and run new + and - wires only to the new hot battery and trim pumps only...separate from all other wires in the boat.

First remove the trim pump Pos and Neg leads from whatever they are connect to on the side giving you the most trouble. . Pos may be running through the perko switch or directly to the battery. Next make sure you have the perko switch turned to battery 1 then remove all wires from battery 2 . You will have to unscrew perko switch from wall of boat to track wires to determine what is battery 2. Next hook only wires from trim pump to battery 2. So now you have a single battery set up on that side to start and run motor and an auxiliary battery to run trim pump. This is only for testing, dont try and make a day of it on the lake as the trim pumps pull a lot of juice and could run down that auxiliary battery if you run all day working the trims a bunch.

Chris

Chris. I'm going to have to start the other engine to get on plane. It only cuts in and out at 4700 rpms under load .................confused

Don't.

Re-run boat and see whuch side stays effected, and will help track issue down further,

When all fixed, better to keep engine's separated and only tie together for a jump start.

Good Point

If you completely isolate the trim pump power, (read ground), you will loose the ground reference for the pick solenoid for the trim pump and render it inoperative. I may be wrong about this but don't think so.
Good luck