My Nikon F4s is in trouble. See attachment.
Upper frame A mode, middle frame P mode, lower frame M mode.
Shot 3 frames each of a scene in "A","P" and "M". "P" underexposes in each frame of the 36exp E6 roll.
Comparing F100 and F4s in S mode yields a F4S exposure time which is much shorter than F100 exposure times ( 1,2 and 4 secs tested because you can "hear" it starting and stopping).

So my conclusions:
1) matrix meter's fine in viewfinder prism as "A"mode is 100% ok
2) shutter's OK as A and M work fine.
3) "computerised" program and shutter priority are

What would you do next? Anyone who's got this problem on his F4/F4s/F4e and got it solved?

I'm considering a KEH body without prism to swap the defect body and have a spare one for parts.

Any answers and tips welcome!