BT losing residual fuel pressure

[marvin]

my BT is losing residual pressure. after shutting down the engine, residual pressure drops from 40 psi to 30 psi in less than 2-3 mins ... down to 0 psi in 15-20 mins. here is what i have:

fuel pressure reg ... replaced 2 yrs ago w/ TOMS adjustable FPR.
fuel pump ... replaced 1 yr ago w/ mr2 gze fuel pump (OEM).
injectors ... cleaned 3 months ago. i was told they have been pressure tested and were fine.

after the fuel pump was replaced and the injectors were cleaned, the problem remained. i pulled the fuel return line after shutting the engine; there was no leak from the FPR but the residual pressure was still dropping.

symptom: hard starting when the engine is warm/hot.

any clues on what else can go wrong or what to look for? thanks in advance.

marvin