Hi Richard,
I use Revell acrylics, thinned with tamiya thinner around 50/50 and spray at 20-22 PSI, thats the pressure that I set on the dial, spray a couple of bursts of air and my compressor dips, so I reset at 20-22 and although it continues to dip when spraying it should return to the set pressure, thats quite normal. When I started I was having the same issue as you, fine spray, stop spraying, when I started again I would get large droplets and few large splodges.
I started adding Winsor and Newton flow improver to the mix and haven't had an issue since. No dry tips, blockages or splodges. I would also suggest trying more thinner to paint as this may help too.
Just did some checking as I remember seeing a mix ratio guide for Ultimate as I have some and found it on the review thread,
http://www.britmodeller.com/forums/index.php?/topic/234956555-ultimate-acrylic-airbrush-thinner-cleaner/?hl=%2Bultimate+%2Bairbrush
The mix chart is down the page a little.
It recommends 50/50 mix with Humbrol acrylics.
Hope this helps.