Hi, Swifty; We've seen this problem many times with engines that use the Variable Valve Timing. Most times, it is old oil, or the wrong weight of oil. This engine should be using the 5W30 oil. In cold climates, 5W20 oil. Yes, many times the sensor/solenoid can be removed and cleaned. It can also be manually tested while it is out of the engine. Fairly common to have to replace the sensor/solenoid. You say that you change the oil and filter every 3 or 4 months? How many miles are driven during that amount of time?
Loren