My first choice for checking exposure would be through the use of reversal film where any gross over- or under-exposure will be patently obvious and immediately negates post-exposure (processing) errors. Negative film has a lot more latitude than reversal and this is where judgement of over- under-exposure can be prejudiced.

As you have repeated tests using all the Zuiko lenses you have, a good idea would be to run a test on a lens you do not have (loan one, if possible) i.e. a Sigma or another, different Zuiko. If the tests return the same fault, the shutter curtain (already a suspect given the age of the camera) and the TTL meter need to be examined. I wouldn't leave it to guesswork especially if at some stage you have an unexpected, important assignment, but to nail the problem run a roll of reversal film through it and beside normal exposure, key in N (normal exposure), +0.5 and +1.0 stop, then N, -0.5, -1.0. Errors from these settings will be quite alarming on reversal film (given its characteristic narrow latitude).