Need some help please

Past weekend we were out and rpm's would drop 300-500 and back up again repeated on its own and/or as I would use trim tabs or raise or lower seats. Thinking the problem may be in combination with another issue not sure.
GPS also turns off when seats/trim is adjusted. GPS is powered off a spare fuse under dash on its own. Tach was flickering pulled harness off connection on motor cleaned and re-connected. Tach is fine now but motor still feels as if it is running on 7 then 8 then 7 ......etc

I feel it has to be an ignition problem or wire connection
I have read that many have problems with MSD I have the MSD 6m-2L. with msd dist. and coil. do they act like this when they are on the way out? Or do they just stop working/no spark period type thing?

502 hp500 built to 540 with 250B&M

Any Ideas Thanks

it sounds like you have a ground issue that may also be causing the missing,in my experiance,when a msd box goes bad it just quits.

+100

Electrical 101, check all grounds first. You will be upset if you start buying parts only to find out it is a ground issue.....and you also would not be the first oso'er to make that mistake.

or a lein condition,,do you have a fuel pressure gauge? try yo keep it at 7 psi..are you running close to the rev limiter?

8psi all day and if anything im running fat for sure.

Was not sure how they go.... but sounds like they just quit. Thanks...

My first thoughts was ground but thought I would toss any and all symptoms out here

I have been bit quite a few times with bad grounds in boats. Even tight connections can corrode between different metals, I have found dielectric silicone on these connections helps a lot in a marine environment. Started doing that with stereo amps & electrical gremlins for the most part went away, hope you find the problem quickly, Randy

MSD

May
Suddenly
Die

Being a die hard racer, I may have some input.
Check to make sure the wire comeing from the distributor is at least 6 to 8 inches away from any spark plug wires, you might be picking up EMI from a plug wire that is leaking voltage. And the obviouse, check all your connections, clean them and a little dab of dielectric grease goes a long way. You can also contact MSD via their web site, they have a forum. Or you can call the technical support line. You will get your best answer to you questions from them, afterall they are the MSD experts.

I had similar problems that took many outings and searches. Culprit was crank sensory lead that was corrouded. Would run fine, then stall, would start then not start. Was very exasperating...

I was having exact same issue with the RPM fluctuating in my boat that I just bought in aug. I gave it a tune up, didnt help. I've got a MSD 6al box and assumed that was it. I pulled motor, more to just inspect its condition and parts. I did find that it had a bronze dist. gear with the teeth wore paper thin. Im not sure if that had anything to do with it or not?? Probably no help to you at all, but thought Id mention it. With all your other issues I would think it might be a grounding thing and could have been my problem to?