I have cable internet modem wierd problem...

I rescently had to reinstall everything on my computer and thus it means alot of downloading, games, software etc. But for some reason whenever I start a major download my cable modem resets meaning power off, and all the lights take awhile to come back on to normal operation levels. It seems that normal browsing,vid streaming, etc works fine, but start any major download like steam or what have you and it resets. It never did this before. I had technician come by and checked my signal and everything checked out he said my signal was low. He fixed it and said that my modem was single channel whearas there are newer modems that have multichannel that provide better stability and stuff.Also it bears mentioning my modem is about 3 years old. So kinda old . Is their any credit to what he is saying or is it server side? Did some admin somewhere get on his high horse and mod me since he saw i was downloading alot? I only use steam and MMO and system software to get stuff, I dont use that pirated stuff. So before I go and put down 60-100 bucks on a new modem am I out of options? I just find it wierd that normal internet isnt affected and downloading is :( Any help would be appriciated thanks.