the RDseries parts go to RFParts.
Getting enough output? your driving the BS170s into class C or even maybe class E, 500mw should be easy.
Using RD15VHF is gross over kill. You do not have enough drive to get more than maybe a watt. Also being VHF
power fet it can and will very likely take off at VHF.
The power variations is most attributable to either the voltage at the top of the bias pot being unstable(unregulated)
changing the bias to the fet and the fet going into oscillation.