Sounds like you're puttin' on too much paint my friend!! My suggestion is to plan on painting the model in *several* light coats rather than one or two heavy coats. Here's a few thoughts:
1. Make sure the plastic has been washed with warm water and soap. There are often chemicals on the plastic from the manuf. process that can cause 'difficulties' for the paint...
2. Put a primer coat on the model. This will provide a nice surface for the paint to stick to
3. The first two or three coats of paint should be VERY LIGHT and almost misted onto the surface of the body. You won't get full coverage of the paint and you'll be able to see the plastic very easily. The paint will be 'speckled', but that's OK. Let it dry for 15-20 minutes and do another light coat the same way. Do this 3 or 4 times until the body is covered in paint and you can't see the plastic any more.
4. Now, start putting on a heavier coat of paint. The heavier paint will 'blend' with the first few coats you have on (speckled) and should flow out nicely. Be careful about putting too much paint on, because it might look good as it is coming out of the can, but about 10 seconds later it will start to pool and run on the surface of the model.
Give it a try and see if this helps. Chances are if you're saying to youself '... just one more pass of the can and it will be PERFECT', then you should stop and NOT DO IT. Let it dry and assess the results before applying more paint.
I still mess up all the time too. It's hard for me to follow my own advice, but every now and then I listen to the little voices in my head and things turn out pretty good...
M.