A great list of things to check when your chain is misbehaving according to Jeremy from Baltimore Burners - 3/22/19:
Size of chain: #25 chain and T8F have smaller links, smaller teeth, so less engagement, less room for alignment error, and less room for tensioning error. most teams run #35 chain or larger. While #25 and T8F are capable of reliably driving a PRS car, the tolerances for doing so are much tighter and they are operating at a higher percentage of their load capacity
Trueness of sprockets: when viewed from the top, do the sprockets wobble side to side as they rotate? sprockets out of true will result in changing alignment as they rotate
Eccentricity of the sprockets: when viewed from the top spinning without chain, does the sprocket have a constant radius? a changing radius will result in changing tension as they rotate
Retention of sprockets: do the sprockets slip axially along their respective shafts?
alignment of shafts: are the shafts parallel? if not, the chain will flex more
Alignment of the sprockets: when viewed from the top, are the sprockets coplanar? If not, the chain will flex more. Check with the motor running, the sideways motion of the chain due to misalignment will be more apparent
Tension of chain: When viewed from the side while running, does the chain wobble around significantly? It is important to manually check this at rest and visually check it at speed, as drive forces may bend axles, brackets, etc
Binding of chain: Take the chain off. manipulate each link by hand. are any of the links stiff?
Spin the drivetrain by hand. are there any spots that have a higher resistance than other spots? This would indicate one of the above problems and will result in a chain tension that is not consistent throughout the sprocket rotation
Sprocket wear: are the sprockets worn or deformed?
Chain stretch: Chain will stretch as it is used. compare your chain with an equivalent number of links of new chain. is the old chain more than 1% longer than the new chain? if so, replace it
According to no actual research the most popular chain size in PRS is an ANSI #35 chain. If you run this size you may find other teams in the pits with replacement parts. (half links, master links, etc.)