Yellow froth, losing coolant

I have a 1998 1.6 Sport A3 and for about 4 months now it has been losing coolant at a very slow rate. By that, I mean that if I fill the coolant up to the Max marker it takes 6-8 weeks to drop below the minimum marker and prompt me to top it up.

I just assumed that it was a tiny leak somewhere and decided to get it investigated at my next service but the other day I removed the oil filler cap to discover a yellow coating on the underside of the cap and around the neck of the hole that you pour new oil into.

I immediately started to put 2 and 2 together and draw the conclusion that coolant loss plus yellow emulsion in the filler cap means head gasket is gone but my garage, who I called the next day, insist otherwise. They said that the yellow coating that I can see (which is about 2mm thick and pretty much evenly coats the bottom of the cap and the neck down towards the rocker cover) is common in modern engines and is simply the result of condensation. If I remove the dipstick, there is no sign of the yellow froth on it.

Of late (for abut the last 2 months), my car has been used Monday-Friday for a commute to my new job. This is about 10 miles in each direction so the engine is just about getting up to temperature as I arrive at work but rarely runs for a long period at normal operating temperatures.

Now, I'm no expert but I can't help but think that the signs don't look good. The only thing that makes me think it may not be the head gasket is that the rate of losing coolant hasn't increased - more mechanically minded people than I have told me that once it goes it gets progressively worse as the gases pass through the gap.

Also, the garage said that this particular model engine is rarely subject to head gasket failures so the stats are suggesting that the likelihood that that is my problem is low.

The only other things worth mentioning in relation to this is that when I first start the car on the cold mornings recently it seems to only run on 3 cylinders for 5-10 seconds - a single gentle blip of the accelerator irons it out. I was hoping to get his sorted, assuming it is likely leads or plugs, at my next service too.

Does anyone have any anecdotal experience or thoughts on what this might be - am I worrying about nothing? Am I about to be hit for a 4 figure bill? Should I get rid of the car now while the going is still good?

Thanks in advance.