Oil furnace/boiler pressure keeps rising...help

[RAYBOMON]

I recently had my yearly cleaning done on my oil furnace but I noted that my boiler pressure keep rising past the 75 psi on the gauge...so far the a valve, the pressure guage (to be sure it was reading correct), and I just replaced the Amtrol 30 expansion tank. Lots of money later, the problem still occurs - I have to keep letting water out of the close system to bring the pressure down. The only thing that I have found is if I shut the valves to the hot water heater (a second zone) the pressure remains at around 20 psi but I get no hot water. Anyone have any suggestion that can help me??