Speed Improvement - Macro Extract Higher/lower Values And Note Time Differences

I have a macro that needs a major improvement in performance. I have a Quad Core 2.67GHz single processor computer and with the current logic it calculates 20 values per minute. I have data sets that can be up to 400,000 data points, which means it will take 333 hours. The attached Sample file has over 9000 point. For this data set it takes 7.5 hours.

The core logic of the macro is to extract what I call “Break Point” into column M and “Time Difference” between the Break Points to column O.

I got the code from this forum (thank you DonkeyOte) and made some modifications. The modified logic does the following:

1) The user inputs the starting cell. In the Sample I use G200.

2) From cell defined in button, It moves down one cell in that column and compares that value to the original cell.

If the value is greater than original cell then there are 2 possible outcomes:

a) move down 1 cell in column and if that value is less than original cell, then extract that value and copy that value to the column M (Break Point value) and copy the Time Difference Value to column O. “Time Difference” is calculated as the difference between the 2 point breaks in Column A. In this case I’ve hardcoded 0.003472222 to get 5 minutes which matches up with each incremental time in column A, but I would like the Macro to automatically calculate that by the difference between Column A values.
b) move down 1 cell in column and if that value is greater than original cell, then move down again, until find lesser value than original cell. Once we find value less than original cell, extract value and copy value to column M and copy the Time Difference Value to column O.

Once lesser value has been copied to new cell, logic is now:
a) move down 1 cell in column and if that value is greater than original cell, then extract that value and copy that value one cell to column M and copy the Time Difference Value to column O.
b) move down 1 cell in column and if that value is less than original cell, then move up again, until find greater value than original cell. Once we find value greater than original cell, extract value and copy value one cell to column M and copy the Time Difference Value to column O.

At any point in the process if we find a value equal to the starting cell, we ignore it and the logic continues. The logic continues flip flopping like this to the last value of column G.

OR
Here's the logic now in reverse I need:

If after moving down 1 cell of original cell, the value is less than original cell then logic has the following 2 outcomes:

a) move down 1 cell in column and if that value is greater than original cell, then extract that value and copy that value one cell to column M and copy the Time Difference Value to column O..
b) move down 1 cell in column and if that value less than original cell, then move up again, until find greater value than original cell. Once we find value greater than original cell, extract value and copy value one cell to column M and copy the Time Difference Value to column O.

Once greater value has been copied to new cell, logic is now:
a) move down 1 cell in column and if that value is less than original cell, then extract that value and copy that value one cell to column M and copy the Time Difference Value to column O.
b) move down 1 cell in column and if that value is greater than original cell, then move up again, until find lesser value than original cell. Once we find value lesser than original cell, extract value and copy value one cell to column M and copy the Time Difference Value to column O.

Again, at any point in the process if we find a value equal to the starting cell, we ignore it and the logic continues. The logic continues flip flopping like this to the last value of column.

Once all the Break Points and Time Difference between Break Points are extracted for each value in column G, the AVERAGE, STDEV and MAX values of column O are calculated in column Q, R and S.

There is a loop that controls the execution of the core logic until the last value in column G. In the macro I’ve hardcoded last row being 9171, but I’d like the Macro to figure out what the last row is automatically.

I know this description is a bit long but I’ve worked for many hours on getting it work properly. I just need some help to make it run much faster. I hope someone can help me out on this. I have the right logic, just need the speed now and I really can’t figure it out.

Attached is a sample files demonstrating showing how the logic to flips back and forth as moving down the column starting in cell G200.

The sample shows the results first 4 loops (Rows 200 to 203). The values in M and O are the results of the fourth loop.

Again, assistance is very much appreciated because I've taken this as far as I can with my limited experience.

EDIT - I can embed the Macro, but it'll probably much easier to actually see it in the sample file.