Never have learned to fix PLL circuits without a 'scope. If the signal at TP10 is too low, the PLL chip has a hard time locking onto the selected channel frequency. Might be what's going on here, might not.
I find that L21 is frequently adjusted way off from its proper peak setting. Since it won't affect either the S-meter or the wattmeter in a direct way when you turn it, a tweaker may leave that one set in some random position after "trying" it to see if it boosted a meter reading.
A 'scope at TP10 is the best way to get the slug in L21 set at its proper peak. Lacking that, a RF Voltmeter would also tell you this.
A DC/AC multi meter won't.
We check this first whenever we see this kind of PLL misbehaving.
73