93 Silverado Skips at High Speeds

I have a '93 Silverado 4x4 with a manual transmission that occasionally skips at high speeds after driving for 2 or 3 hours. It does not skip at high speeds (> 60 mph) until after driving for a few hours, and then the engine seems to skip once every minute to minute and half. Any thoughts? A friend suggested that it might be electronic advance problem.

How many miles on your car? Has all the recommended maintenance been done?

It has 168,000 miles one it, but only 10,000 on the engine. yes the recommended maintenance has been taken care of.

If you have a tach did you notice if the rpm goes up or down?