Parasitic oscillation using high Ft transistor

The active mixer does not work well above a few MHz of RF/LO, the output level can drops from -14dbm at 5MHz to -30dbm at 27MHz, so I replaced all transistor, these PN2222 general purpose transistor, to RF transistor BFR93A (Ft = 5GHz), surprisingly, even the inputs are all terminated in 50 ohm, there are a ~1.2GHz -20dbm parasitic oscillation at the output! If I could not get rid of this oscillation I won't be able to continue with any other test! I tried to added some smaller value supply decoupling capacitor but it doesn't help. The problem may come from the board layout. When I put finger on the board, it's not easy to disturb the oscillation, it just shift down the frequency a bit when finger tap on the upper side transistors. May be I should change to a lower Ft transistor.