Any radar sets in operation when you have the problem? Mid-range radar emissions can screw up GPS reception (seems to happen more with older antennae), and high power emissions can kill the GPS antennae.
You might try cleaning the top of the module with a mild solvent. Probably won't help, though.
Other than those, I know that some of the Lowrance GPS modules were a bit sketchy in operation. They have had quite a few odd problems, including some that happen as you describe. Unfortunately, there is very little you can do with these things as far as troubleshooting goes - swapping is about the only method; so it makes it pretty tough to ID the actual prolem cause.