Fluctuating fuel pressure?

So.. for a long time now I have been somewhat uncertain about my fuel pressure.

See sig for specs/mods.

Occasionally my Kenne Bell in-car fuel pressure gauge has showed sudden drops/fluctuations in my fuel pressure, falling from approx 39 psi down to 30 or 20 or even 0 and then up again and down and so on - however only when idling or coasting or highway driving - never under load.
In the beginning it scared the crap out of me, but I never noticed anything different enginewise during those periods of fluctuations, and after awhile the fuel pressure would stabilize at around 39 psi according to the gauge.

Now I have just completed a cylinder head swap - new Trick Flow street heads, new Summit MAF calibrated for 30 lbs injectors, new Trick Flow 30 lbs injectors, new Aeromotive adjustable fuel pressure regulator, new Trick Flow 190 lph in-tank fuel pump, new TB and EGR plate.

So, upon start-up I adjusted the fuel pressure to read approximately 39 psi when vacuum hooked up, and the car idled nicely.
The first test drive went ok, but now according to the fuel pressure gauge the fuel pressure fluctuates more than ever (say it fluctuates half the time or more).

I do not get any relevant error codes.

Is it just my KB pressure gauge being a POS (which wouldnt surprise me the least) or can there be something else, like a partly clogged fuel line or fuel filter (I am going to replace the fuel filter on Monday)?

So far I am not impressed at all with any increase in performance, but maybe it all improves with some further tuning.

But whats wrong with my fuel pressure??

I'm getting seriously frustrated and fed up with this car.

jrichker said:

Look very closely at the fuel pump relay, & relay socket. You may have some corrosion or overheating issues with the wiring connected to them. Being the relay is under the hood, it gets exposed to a lot more heat than the eariler model Mustangs.

Click to expand...

Thanks - the fuel pump relay looks pretty good, but I gave it a little cleaning anyway. Didn't help though.
I also cleaned the positive and negative contact to the fuel pressure gauge sender, but the situation is pretty much unchanged.

I am only getting one error code, code 66 (see separate post) but I dont think this has any relevance for my fuel pressure problem?

Jaswir said:

i would try another gauge just to make sure it didn't crap out

Click to expand...

Yea, I'll do that as soon as I can find someone with a spare - I am broke now...

However, what bugs me is that I can't feel any difference in driveability at all when these pressure drops occur - I mean surely I would somehow notice a change in how it runs when the pressure drops from 40 psi to 5 psi?
Or wouldnt I?