If the other baseboards & radiators on the string are all still putting out heat, this system is probably plumbed with "monoflow Tees" bypassing each radiator, and an air-lock/bubble has developed in the baseboard section. To know for sure you'd have to air-purge that loop.
If there is a valve on the main loop pipe in the middle of the bypassing element to the radiator that can be closed, forcing the water to flow through the section of baseboard that alone may be enough to stop the symptom, but if there's that much air in the system it may recur (the same place or elsewhere). Most hydronic systems have an "air scoop" and vent valve to get rid of residual bubbles after air-purging the system by opening it up (best near the high point of the system) and letting massive amounts of water flow for awhile.
A system pressure of 20lbs is a bit high for a simple 1-2 story (12-13 psi is enough). If the pressure is too low it can let corrosive amounts of oxygen into some systems (8psi is an absolute minimum), but there's no advantage to going higher pressure than necessary. On 3-4 story buildings 20psi may be necessary to keep the pressure high enough at the high point of the system, but many boilers can't handle more than 30psi. Some ultra-modern low-mass boilers need 20psi for optimal performance, but I'm assuming you have a traditional cast iron unit. The system pressure can often be set by tweaking a screw on the autofill valve, and system pressures can rise if that valve is leaking &/or the expansion tank has failed (or needs to be air-charged). Read up on it before making any adjustments (or hire a pro.)
The middle of winter isn't the ideal time for a DIYer to learn the joys of air-purging hydronic systems, but you know better than me what your capabilities are. No rocket scientists are out there wasting their careers purging heating systems, but some forethought and patience is required (and sometimes some plumbing skills.) If you're lucky there will be purge valve (or multiples) on the system, preferably near the highest point on the system.