How can these two phenomenon be related?They are not related, they are two separate issues.
RCPrince correctly addressed the issue of crosstalk between inputs that is occurring in the preamp.
Mlsstl offered some good suggestions concerning the issue of rfi from the cdp affecting the tuner. I would add that if you are already using an external antenna, it should preferably be one that is connected to the tuner via 75 ohm coax over the entire run, and not 300 ohm twinlead plus a 300 ohm to 75 ohm adapter at the tuner input.
Most probably the reason for the different behavior above 98.1 MHz compared to below that frequency relates to the fact that 98.2 MHz is exactly 10.7 MHz above the bottom of the FM band (which is 87.5 MHz, although frequencies below 88.1 MHz are not widely used). 10.7 MHz is what is known as the IF (intermediate frequency) that has been used in all FM tuners since the 1940's. What is known as a "local oscillator" circuit in the tuner generates a signal exactly 10.7 MHz below the frequency of the station that is tuned in. The two signals are then mixed together to "beat" the station's signal down to the 10.7 MHz difference frequency, which is then amplified and processed further by the IF circuits in the tuner.
So for stations above 98.2 MHz the local oscillator is putting out a signal within the FM band (above 87.5 MHz), while for stations below that frequency the lo frequency is below the FM band. While I can't offhand formulate a complete explanation of what may be going on, I mention all of that simply to make the point that it is very conceivable that behavior for stations above the 98.1 MHz frequency you mentioned could be different than for stations below that frequency.
Regards,
-- Al