When my 1991 geo storm warms up the idle skips around.... it doesnt stall out it just sputters and gos from high to low idle

i cant understand that but got BAD idle.
there are different idle speeds, for hot ,warm and cold engine.
and the PCM (ECU) regulates idle with valve called ISC.
bad idle can be 1 of 2 things.
ISC sticking.
or engine runs so bad the ECU can NOT control idle.

clean the ISC, if not that
then its a big job, to find true cause. ASK !
one is finding air leaks.aka, vacuum leaks.

triple posted back up and read answers.

vacuum line is not connected properly or line may be damaged cut or burned. minor issue should be able to hear loud air suction noise

u need a map sensor

Haynes manual shows this for that problem, but please take it with a grain of salt and dash of peper:

1. Idle speed incorrect
2. Fuel filter clogged or water and impurities in fuel system
3. Distributor components damp or damaged
4. Faulty emissions system component (talk about vague :)
5. Faulty or incorrectly gapped spark plugs
6. Faulyt spark plug wires
7. Vacuum leak in intake manifold or vacuum hoses
8. Valve clearances incorrectly set

That last one is possible IF the new timing chain/belt was installed a tooth off.

All info is from a Geo Metro/Sprint Manual, but it may help.

When you say "fixed", do you mean that you put the new belt on it?
If so, you probably missed a tooth in the belt placement. This usually happens when you wrap the wrong way during the procedure and the marks don't end up lining up.
Also check for other things like pinched wires or something disconnected that didn't get put back in the right spot. (vacuum lines?)

Try this
Here are the most common causes of surges, stalls at stops, low erratic idle speed, rough idle and engine hesitation, it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Fuel injectors may need cleaning.

I feel the codes may be the answer here, idle should be around 8-900 rpm. There are a lot of possibilities for this kind of issue and the codes would be a great place to start.

maybe try replaceing the ingnition modual. that part can do a lot of weird things to these cars. its located inside the distributor cap, really easy to do. about $40 and 10 mins or less. sometimes as they start to fail they do odd things sometimes but not always