Thanks Chuck. I sent you a PM.
I would check the fuel pressure with engine off and key turned on, then with engine running.
On my '72 SL, I found that I could eliminate the hot start problem by increasing the fuel pressure to 30 psig with engine off. (It goes up to 32psig with engine running presumably due to the back pressure in the return line). You do the adjustment with the fuel pressure regulator at the back of the engine on the fuel rail. I installed a tee into the the rubber hose going to the cold start injector. Just used a cheap gauge I happened to have here - it's 0-100 psig so not to precise, but idea was to bump up pressure a few psi - it was reading 27 psig.
You should also turn everything off and watch pressure gauge. It should hold pressure within a psi or so for several hours (specs are in manual). If not, it could be that an injector or two are leaking fuel into engine.
Increasing fuel pressure may cause the engine to run rich, so the MPS might need adjustment to bring CO in exhaust back to spec. Or maybe just twiddling the knob on the ECU will do it? I have no way of measuring CO, so have not done this yet.
One other thing I notice, is that the problem "almost" re-appears when the tank is low on gas. Maybe this affects fuel pressure? Anyway, I am changing the fuel filter out today in hope that that will help.
Prior to this, I did all the other things I read on the web and none of them made any difference. Only one I did not try, was changing check valve on fuel pump. Can't find reference, but this should also show up as a bleed down of fuel pressure.