  We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift
radio-intermediate PG quasars (RIQ) with flat and variable radio spectrum (III
Zw 2, PG 1309+355, PG 2209+184). Their radio-to-optical flux ratio (R) is
slightly lower than the average R for steep-spectrum quasars, but their radio
spectral properties are those of core-dominated quasars. It was proposed
previously that these sources might be relativistically boosted jets in
radio-weak quasars. Our VLBI observations now indeed confirm the presence of a
high brightness temperature core in all three of these objects --- two of them
have lower limits on T_B well in excess of 10^10 Kelvin. Moreover, we find no
``missing-flux'' which means that basically all the flux of these quasars is
concentrated in the compact radio core. As the total radio flux is already at
the low end for radio-loud quasars, we can place a strong limit on the presence
of any extended emission. This limit is consistent with the extended emission
in radio-weak quasars, but excludes that the flat-spectrum RIQ reside in
typical radio-loud quasars. The observations therefore strongly support the
idea that relativistic jets are present in radio-weak quasars and hence that
radio-loud and radio-weak quasars have very similar central engines.
