87 944 16v itermittant rpm drop
 

bought this car after the prior owner had the timing belt break. just rebuilt cylinder head and replaced gasket water pump and thermostat bled air out of system. now have two problems. Temp sits at 3/4 mark after warm up and stays there until driving and stuck at lights goes up to red marker and makes its way back to 3/4 mark. now also drops rpm to 400/500 occasionally now and again stops completely but will re start. highway speed runs like a dream temp just below 3/4
any ideas please. I was told it didn't have these problems until the belt broke and it bent ten valves.

Maybe a vacuum leak somewhere? If it's not that that, maybe a problem with the ISV?

Do the fans turn on when the car is hot?

Multiple symptoms make it tougher to diagnose. But i think you have two separate issues. what you can consider here is that a problem seems to occur (idle dip), then the problem is usually resolved. I'd refer to sausage's post, because it's sounds like a vacuum leak, or especially ISV... or temporary fuel shortage.

Is the idle dip a sudden drop, then a strong recovery?
Or more of a stumble to lower RPMs, then a stumble to recovery?

3/4 on the temp gauge seems high to me. My S sits just above the first mark basically no matter what. Even hard driving in 90+ degree heat won't push it higher than halfway.

Are you sure you got all the air out? Is the temp sensor functioning correctly?

Agree with SloMo28.

You need to bleed ALL of the air out of the system.
For some reason, I find it harder to get all of the air out on the 16 valve head.

Slow way to do it but works:
1. Engine off. Cold.
2. Remove radiator cap from expansion tank. Tank should be at minimum level or higher. Don't let it drop too far below the minimum level.
3. Loosen the bleed screw a little.
4. Squeeze the lower radiator hose and hold. You should hear gurgling and air, possibly some coolant out the open bleed screw. You can pump it several times before closing the bleed screw to force more of the air to the top of the system. Experiment.
5. Tighten the bleed screw.
6. Release the lower radiator hose. Probably hear some gurgling. It should draw some coolant from the expansion tank back into the system.
7. Repeat until no gurgling is heard or only coolant comes out the bleed screew when squeezing the lower radiator hose.

The engine coolant temperature should hold under almost any condition to the first white mark, plus or minus about a pointer's width when it is running and no air in coolant.