Did you ask Sciex engineers? 

If it's not connected with detector malfunction or electronics, try the following:

without restarting the MS clean the orifice, be sure that the hole is open. 

Assuming you don't have a service contract, it sounds like it could be "charging" of your quadrupole rail.  Have you cleaned the front end (behind the sampling and skimmer cones)?  If you clean the front of the first set of quads (carefully, do not risk lint getting into the system), you might avoid needing an engineer to pull the rail.

I did a manual tuning using PPG standards, and all the results were within the criteria of sensitivity, resolution and mass shift, except the Intensity of the masses 175, 500 and 906 in positive mode in both Q1 and 906 in Q3, where the signal's intensity were below the accepted limits. I'd also done a front end cleaning and after it i run PPG again with Auto calibration (instrument optimization) and the 906 signal intensity was still below the criteria.

any other advises, or tests to perform ?

Run PPG in positive mode for 10min and record the intensities for 175, 500, and 906. Then stop acquisition and switch the machine to negative mode using same method as above, Now run the PPG for 10-15 min. Stop acquisition again and switch positive mode and start acquisition. If you see a significant gain in the 175,500, and 906, this means the quad/quads are contaminated. Also if you are running nano spray, check if the orfice heater is heating, as this could cause drop in intensity as well. 

You can increase intensity by adjusting method parameters, but to understand the problem check the entrance: maybe something sticks to it during standby, because decreased intensity could mean either lower amount entered to the instrument or lower amount is detected.