Each time you increase your Fever level, you will move up the ladder of Fever patterns. Each pattern has a different distribution of rings and point bubbles. The higher the level becomes, the more those patterns shift towards point bubbles instead of rings. This will eventually noticeably decrease on how many rings you can collect per run. If you trigger Fever more than once in a run, you can see what the next pattern looks like. If needed delay upgrades until you have collected a large number of gold rings.