Are there any initialization strings to force a V.90 connection for this modem? Where do I put them in Dial-Up Networking?

Along with other factors (e.g., line noise, server conditions, amount of other traffic online, operating system or communications software problems), the problem(s) you're experiencing may be influenced by what modem init string you're using -- if any. Try the following modem init strings (whichever works best for you) by highlighting your Zoom 56K modem's name in Control Panel-Modems and pressing Properties-Connection-Advanced and entering these strings one-at-a-time in the Extra Settings field:

ATW2S7=150S109=2
AT&FS109=2S7=150
AT&FS109=2\N3
ATS109=2
AT&F&C1&D2\N4\A2S109=2S7=100