Question: Presently I am a married 55 year old man married with two grown kids. One of my term life insurance policies expires soon and they have offered me a policy that will never increase in price for the rest of my life. The premium is quite a bit higher and the amount is less than what I presently have.

I do invest pretty heavily but lately the market has been very unpredictable. Would I be better off to perhaps take those additional monies and invest it?

Answer: 
Life insurance is not an investment -- by definition, since the companies need to take a profit out of it, the average amount paid in exceeds the amount paid out, yielding a negative rate of return.

Get life insurance if your death would cause severe financial hardship for someone. If you have sufficient savings that your wife could recover and move on with her life without hardship, and your kids are grown, you probably DO NOT need life insurance.
If I were in your shoes, I would invest conservatively fully aware that for the next few years the stock market is going to be depressed, but then again, don't take that as advice. Every situation is different, weigh the pros and cons carefully and if required, consult a qualified professional.