I seem to have hit a dead spot in increasing my FTP - may be near my max??? I'm 45 and weigh 135-140. My current FTP is 260 and has been at this level pretty much all year.

I'm thinking of changing things up a bit and doing intervals/repeats by holding 300 watts for as long as I can. Then rest for 5 minutes and do the same thing again - etc, etc, etc. Currently based on my Strava power curve I've held 300 watts for 8 minutes.

What are your thoughts on the effectiveness of this method as I haven't heard it before I'm thinking there are some negatives that have escaped me?

There are two schools of thought - push your FTP up from below, or pull it up from above.

10 minute intervals starting at 110% of FTP seem to work well for me with 5 minutes rest between, noting that's at or close to time to exhaustion for me because I don't have much in the way of anaerobic reserves.

Stephen Seiler's test subjects had the biggest improvements in lactate threshold and VO2max with 4x8 intervals "as hard as possible" with 2 minutes rest between them; twice a week with the rest of their training time spent below their aerobic threshold.

He suggested 7-10 minutes with 2 minutes rest in his post from January 22nd, 2015 on slowtwitch

Seiler may be the most visible polarized proponent, where 20% of sessions are above the anaerobic threshold and 80% below the aerobic threshold (generally in Friel's zone 2).

I agree, the 10 on 5 off are great. According to Seiler, the goal of intervals is to accumulate as much time as possible during the workout at high intensity. By breaking it up instead of doing it all at once, you end up spending more time above threshold.

So 4 x 10 would give you 40 minutes at 110%, which is more digestible than a single 40 min attempt.

3 Workouts to Raise Your Functional Threshold Power | TrainingPeaks

I've been doing several of the workouts suggested with no real impact, but was specifically asking about 115% efforts for as long as one can go with 5 minutes rest.

I may give it a try. I'm thinking I'd get a good quantity of time over FTP with the hope of increasing time I can hold the 115% over the training period.

I predict 8 minutes at 115% (using power). Let us know how it goes.

My FTP is 330 (estimated... 349 in 20 minute test) and I'm hoping to clear 400 by the end of the year. Never trained with power or done intervals before last month.

My "trainer" (cat1 friend) has me doing mostly 3 minute power intervals. Supposedly they bleed over more into other time durations. That, and doing 20 minute intervals sucks. Doing shorter ones will make it harder to burn out on the training program