That's a weird one.
Only thing I can think of is perhaps interference from the alternator as this starts charging at about 1000 rpm?
To rule this out, you could temporarily remove the alternator drive belt.
Also ensure you're got a good ground on the radio and on the antenna.
There might also be an antenna trim adjustment screw to be adjusted for best signal reception.
Maybe try this new radio in another car if available? Or try a known good radio from another car in you car?
Back in the (good) ol' days, it used to be standard practice to have to fit interference suppressors to various electrical devices to prevent electrical interference affecting the radio - these were capacitors of various capacitance values that had to be attached to dynamo/alternator, spark plugs/ht leads, ignition coil, wiper motor etc. plus in more difficult cases, bonding straps and inline chokes (inductances). (glass-fibre bodied cars were the worst). Remember your car was built in the same 'good-'ol-days'.
