C4 shift behavior

Its never bothered me but got curious about it. 69 Coupe 351W, C4 auto. C4 is rebuilt with less than 3k mi. Its only a summer, weekend driver. Sometimes when I put it in drive or reverse, it can take up to 5 seconds for it to engage. It doesnt do it all the time. Just sometimes.

Could it be fluid level and/or something else mechanical?

Thanks