Rpm cannot keep steady

You have confused me a little.

Am I right to understand that upon starting the engine it runs fine?

When the engine warms up it has erratic idling?

Now you have not mentioned but upon my assumption when the engine is under power or above idle speed you have no issues other than possibly lacking  power?

Your issues could lie with weak emissions (check and adjust fuel mixture as necessary), false air leak from the idle circuit or a dislodged vacuum hose at the back of or under the inlet manifold to ward the bulk head side.

Pedantically,  fuel adjustments ideally should be made ensuring good spark plugs and good gap work, correct ignition timing, good air filter and most importantly correct valve clearances this element is so over looked by owners and garages! My suggestion for checking and adjusting the valve clearances should be done on a stone COLD engine.

It is possible to set them warm but this requires skilled and experienced hands finish the job whilst the engine is within its warm range. No good if the if starting setting on a warm engine and finishing off when the engine has cooled down. Effectively the whole adjustment would be wrong. 

So in summary check for false air leaks in the induction system then have the valve clearances check and adjusted as required you may want to do compression test before after valve clearance and note the difference if at all. 

Look forward to hearing from you.