Ignition problem

I have an old but good 91 Toyota Camry wagon, not worth much to sell but
worth a lot me as a useful workhorse.  It has an intermittent problem
that I haven't seen for many months, and the fact that it's resurfaced
makes me think it might be temperature-related (it's mid-summer here,
although only about 20-23 deg.C).

The typical symptom is that I drive somewhere, park, come back to the
car an hour or so later, and it will not start. (On a couple of
occasions the engine has stopped when I was driving at a low speed, e.g.
coming up to a traffic light.) The battery is good, the engine turns
over, but no spark (I've checked this using a timing light with a pickup
on the spark plug wires.)  In almost all cases it will start after 10-30
minutes, and run as if nothing had happened.  On one occasion it didn't
start even the next day, and I had it towed to a mechanic.  He couldn't
find anything wrong (not surprising since it was working by the time he
got it), but replaced the distributor on the off-chance.  Of course it
made no difference.

I've been over every obvious part of the ignition circuit, but
everything tests out as OK.  The error code (in light flashes) is 43,
which according to the manual is:

Starter signal.  No STA signal to ECU when vehicle stopped and engine
running over 800 rpm.
1. Main relay circuit
2. IG switch circuit (starter)
3. IG switch
4. ECU

I'm beginning to suspect a faulty relay.  The manual has the circuit
diagrams, but it isn't obvious which part is the "Main relay circuit".
I presume it includes the section labelled "Power source and starting
circuit", but possibly also includes the EFI main relay, and maybe some
other stuff.  The circuit isn't really that complicated, but it's often
unclear to me where the physical components and connectors are on the car.

I just had a look at the fuse/relay box next to the battery, after
running the engine for a few minutes.  Some of the relays have plastic
cases, some metal.  In attempting to pull the relays out, I discovered
that a couple of the metal-case ones are very hot - too hot to hold.
These are labelled Fan no.1 and EFI main.  Is it normal for these relays
to get very hot?

Thanks
Gib