I want ngFor return just one random record (using InfiniteScroll), its possible?

I do not know if this would be the best approach, but let me explain what I'm doing.

I have a mobile application with ionic and angular. On it I have a ngFor that takes data in an observable and normally displays. I put a condition that every 5 records, based on the ngFor index, excute a new ngFor in another observable with ads.

I would like this ad ngFor to return only a random record using a PIPE, but as I use an InfiniteScroll whenever it loads more data it remixes all previously loaded ads.

Is it possible or would it be a better approach?

Heres de template snippet

<div *ngFor="let item of produtos | async; let i = index">        
  <ion-card >
    <ion-item>
    {{ item.name }}
    </ion-item>
  </ion-card>

  <div *ngIf="( (i % 4 == 0) && ( x > 0 ) )" >    
    <ion-card >
      <ion-item *ngFor="let ads of allads |  onerandom" >
        <p>{{ ads?.adsCompany }}</p>
      </ion-item>
    </ion-card>
  </div>
</div>

Heres the PIPE

import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
    name: 'onerandom'
})
export class oneRandomObject implements PipeTransform {
    transform(values): any {       

            var myrand = Math.floor(Math.random() * values.length);
            var temparray = []
            temparray[myrand] = values[myrand];
            return temparray;
    }
}

I do not know how I can fix it to not remix the ads I've already loaded.