Do you have any stored trouble codes? Does the CEL perform its self-test when you cycle the key to on? I'd recommend testing the IAC to rule it out, but I had a failing one that passed these checks and was causing my car to stall out when I would come to a stop. Replacing it solved that issue.

Some information compiled from other's input in the past.
Toyota Idle Air Control Valve
Jump terminals TE1 and E1 to see if the engine rpm is kept at 1,000rpm for 5seconds, and then returns to normal. If not, check resistance between B, and RSC/RSO for 17-24.5Ohms (Cold) and 21.5-28.5Ohms (Hot). If you remove your IAC for this next test, the gasket will need replaced. Once it's removed you apply 12v to the IAC in two steps. + to the middle prong, - to the top prong. The IAC should open or close. Then apply the - lead to the bottom prong and it again should open or close. One is open, one is close but I forgot which is which. The part number for the IAC gasket is 22215-74400 if anyone is looking for it.

 

TY for responding. That some great stuff to test!
Car cranks right up when the CEL is bright upon key on, only no starts when flickering.

No stores trouble codes just rapid CEL flashing when jumping Te1/E1.

I'll do that IACV test and report back. I have the original IACV off the car so should be easy to test that one.

I feel like that flickering CEL/clicking IACV is a symptom of a different issue altogether, related but only a sign, but a very specific one!!

 

Likely a power or ground issue.
CEL is powered by the integration relay and appears to be grounded by the PCM. There's a 10A gauge fuse which feeds the integration relay which is what powers the CEL in the cluster, the 10A gauge fuse gets it's power straight off the ignition switch terminal 4 black/yellow. The ignition switch is powered by fuse AM2 which draws off the battery, powered at all times. It passes through a junction box in the left kick panel. I'd be suspicious of the ignition switch given the age of the vehicle. Try to get a voltage reading on power in and power out of the switch when it's acting up and see if cycling the switch or jiggling it affects it any.
There's also an important ground at the rear of the engine, G115, that grounds the PCM and the combination meter which I believe is just another term for the gauge cluster.

 

Checked/cleaned all grounds in engine bay.

IACV no longer clicking but flicker CEL still happening and causing no start. After whatever signal it is/isn't getting decays after 30 seconds light is bright and it will fire right up.
Going to test IGN starter switch next!

 

Looks like when I wiggle the wires to the IGN starter switch the flickering CEL turns off.
I replaced the starter switch with another and still have intermittent flickering - seems like wire harness connection at the contacts.
I have cleaned with contact cleaner and reinstalled... We'll see how it goes!

 

The starter ignition switch on the back side of the lock cylinder is accessible once you remove the AC duct.

 

Post some pictures of the old switch contacts and the connector. I suspect you have some terminal tension issues with the contacts, if there's heat discoloration that's a sure sign. Often you can use a small pick to lift the contacts back into tighter tension.

 

Upon inspection of the male terminals, all looked evenly colored and no obvious marks. Need to get back under there and look at the plug