John,
There are several different things that could give you these symptoms!
The first that comes to mind for me after hearing / reading your description of the symptoms, is an unloader problem. Some times an unloader valve will malfunction / stick, causing partial bypass when trigger is pulled, (it usually resets when you shut unit off and then start unit up with trigger open, but the problem comes back as soon as you release the trigger and then pull it again!
Next time this happens while gun is open pinch off the bypass line with a pair of pliers, if the pressure increases you have an unloader problem. ( This will obstruct bypass forcing water to go elsewhere such as your trigger gun.
100 hours on unit (not many) can you get assistance from the distributor/ or whomever you bought machine from?
If you need my advice you may call me in the morning between 8:30 - 11:30 AM PDT.