Malibu :: 1999 - Overheating When Driving

I drive a 1999 Malibu 6 cylinder M 3.1L, and this past week it has had a big issue. I noticed when driving to work that it started to heat higher than normal, then drop back down to a normal temp. It kept getting worse and almost overheating, so I went and bought a new thermostat (as I was told that was probably the issue) and a new radiator cap since I knew mine was old and leaked a little bit. Before putting the thermostat in, I had a neighbor look at it, and found some gunk in the coolant tank.

I took it in to get a coolant flush, but when I told the guy at the shop what I found, he told me that because these Chevy's used dexcool, if any other coolant was used, it would gel and therefore start clogging my system and that a flush would do nothing, and that I was looking at a repair that would be more than I was willing to pay.

My question for all of you is should I still go in to get someone to really look at it, get it flushed and put the new thermostat in? Or is the situation that the last mechanic presented probably the issue? I'm pretty broke, and don't want to spend money that I don't have to. But, if $100 will allow my car to run for awhile longer, I would rather do that then have it sit and die while I take the bus.