I would argue that there is no conspiracy. Aspartame is probably one of the most studied supplements, with 100s of studies demonstrating its safety. Systematic reviews of the most rigorous clinical trials confirm this.
Given the increasingly litigious nature of the legal fraternity,a conspiracy involving all the National Health Regulatory Bodies, the industry and clinical researchers over decades seems highly unlikely.
Phenylalanine is an essential amino acid, used by the body to create proteins. It is naturally occuring in breast milk.
Phenylalanine is bioidentical, whether it comes from humans, cows, or any other mammal.
There are some specific issues - Those (rare) individuals who suffer from a condition known as PhenylKetonUria (PKU), an autosomal recessive trait. Such individuals lack an enzyme necessary to break down phenylalanine. This can lead to a build up of phenylalanine to levels sufficiently high that they can damage the Central Nervous System.
Aspartame has also been implicated as a migraine trigger. That having been said, there are many food and related products that are also labelled as migraine triggers, ranging from red wine to coffee to chocolate to any number of other, common foodstuffs.
Another area of research at the moment is if there is any link between the use of sweetners and obesity - but its a complex issue. The clinical evidence would suggest no direct neural or metabolic effect, but it is possible that those who are having problems controlling their weight may overcompensate in their diet, the reasoning being that they are being "good" because they are consuming healthy diet drinks.
It can be very easy to associate a particular system with a specific product - this is often an unconscious bias. because you are looking for the association.If you seriously wish to get to the bottom of these bouts of heart palpitations, you need to approach it seriously. Keep a diary - when the palpitations start, when they stop, any other symptoms. Frequency of these attacks. Keep a record of all you have eaten and drunk during the day leading up to the attack. Don't put off the record keeping - it is easy to forget items you have consumed.
A review of such a diary by your GP would be of great benefit in identifying the most probable cause of your palpitations.