Engine starts racing erratically after a 20 minutes of riding. Help?

Here is the problem I’m facing with my 2007 Harley Sportster XL1200, step by step:I turn on the ignition, flip the kill switch to ‘on’ and wait for the fuel to prime (the bike makes a 2-3 second squealing sound, it has always done this since I got it). After a few seconds, I start it up. It turns over, but sometimes there is a tiny pause (almost like the crank is reluctant) right after the first ignition sound. I make sure to keep the starter button held, so that the bike turns over properly. The engine revs high for a few seconds before settling into its idle RPM.After it has started up, I wait for a good 30 seconds before I put the bike in gear and set off. Usually, a minute into riding, the engine hiccups (misses a fire) exactly twice, with a gap of a few seconds between them. I assumed that these hiccups were related to engine temperature, as they almost always happen exactly in that time frame. None after that.The bike runs perfectly alright for the first 15-20 minutes of highway riding, or 20-25 minutes of city riding. The engine goes back to its idle speed when I close the throttle and there are no hiccups, loss of power.THE REAL PROBLEM: As I get off the highway and begin slowing down, the engine begins to idle erratically, suddenly dropping in RPMs and picking up of its own will. This results in jerky power if the clutch is in. If I come to a stop after this, with the bike in neutral or clutch disengaged, the engine begins to rise in RPMs without any input from me (the throttle is not faulty, it snaps right back and functions as it ought to in the first 20 mins of riding). The RPms rise until I’m embarrassed at the traffic light, conscious of people wondering what the hell I’m doing (I have loud pipes).Even if I speed up after this, the engine runs normally when being throttled, but while coasting, it doesn’t settle into its idle, and give me continuous power depending on what RPM it’s running at. If I coast with the clutch in, it revs even higher due the lack of transmission load.The guys at my local HD dealership have told me there is no problem with the bike (read: they’re incompetent to figure it out). They cleared few O2 sensor error codes for me when I took my bike to them.Few observations that might indicate the problem to you folk: -there are bubbles in the oil sump when the oil is hot. Oil has ran 2k miles -the O2 sensor codes had indicated faulty readings, although the dealership said they were alright -my fuel tank vent line is torn (idk how big this problem is) -killing the engine and restarting it sometimes solves the high RPM, but only sometimesCan’t think of any more issues.If someone has had a similar issue, please let me know what solved it. Thank a lot in advance.TL;DR bike idles erratically high after a 20 min highway ride, won’t settle down to a clean low speed firing. 2007 Sportster 1200. via /r/motorcycles https://www.reddit.com/r/motorcycles/comments/ea2ac7/engine_starts_racing_erratically_after_a_20/?utm_source=ifttt